And it is truly roomy, with back-seat headroom, legroom and hip room that rivals that of a Mercedes S-Class sedan or any of the other cars in this class.This surprised us.The other thing that surprised us was its refinement and smoothness.It's impressively smooth and serene when cruising along like a luxury car, which belies the world-class handling and performance available whenever the driver starts pushing hard on the pedals.

Beyond having four doors, the Panamara is different from traditional Porsches.Unlike the 911, which has its engine mounted in the rear, the Panamera has a more common front-engine design.It is offered with two flavors of V8 power.Panamera S and Panamera 4S models use a 400-horsepower V8, while the Panamera Turbo model features a 500-horsepower turbocharged version of the same engine.Both engines are mated to a new seven-speed automated manual transmission Porsche refers to as the PDK.

The Panamera S comes with rear-wheel drive, while the 4S and Turbo have all-wheel drive.

All Panameras are fast.The normally aspirated V8 in S and 4S models has plenty of power at any speed, and can launch the car from 0-60 mph in as little as 4.6 seconds.The Turbo cuts that time to 3.6 seconds, with little if any turbo lag and a rush of power that pins you back in your seat.Those numbers don't tell the whole story, however.The Panamera S feels lighter on its feet than the Turbo, and is in some ways more entertaining to drive.The 4S falls between the two.

Slightly smaller than a BMW 7 Series, the Panamera offers the sporty performance of a world-class sports sedan with the comfortable ride and refinement of a luxo-cruiser.A lot of engineering went into the car to make those extremes possible.To achieve the desired balance, Porsche built the body from lightweight materials and placed the engine low and as far back as possible.Porsche also used two forms of adjustable suspension to turn the ride from soft but stable to racetrack-ready.

The most luxurious Porsche ever, the Panamera is quite well appointed and equipped.Leather upholstery is standard, as are a choice of carbon, aluminum or five varieties of wood trim.The fit and finish and quality of the materials rival that of any competitor.

Space is impressive, too.A standard full-length center console divides the car into four distinct and comfortable seating positions.The feel from the driver's seat is much like that of the 911, only higher off the ground.The rear seat has enough head room for an NBA point guard and plenty of leg room, too.All of the seats are supportive without being too firm or too deeply bolstered.With this much room, the Panamera would work just as well as a chauffer-driven vehicle as it would a driver-oriented sports sedan.

The hatchback design makes the Panamera useful as a family vehicle, too.With the rear seats up, the rear cargo area is as roomy as most trunks, and with the seats down the Panamera has as much cargo room as a Subaru Impreza hatchback, which is a lot.

Bottom line, the 2010 Porsche Panamera is a great way to expand the Porsche lineup.It performs well on the street and the track, and offers enough passenger and cargo room to make it a no-compromise luxury sedan.Put simply, the Panamera enters the market as one of the world's best luxury sports sedans.Be careful with options, though.As with any Porsche, they can get awfully pricey when you start checking off the boxes.

Panamera S ($89,800) and 4S ($93,800) come standard with leather upholstery; automatic climate control; interior air filter; eight-way power-adjustable front bucket seats with driver's seat memory; heated front seats; split-folding rear bucket seats; tilt/telescoping leather-wrapped steering wheel; cruise control; power windows, locks and mirrors; power rear liftgate; Homelink universal garage door opener; anti-theft system; 11-speaker, 235-watt AM/FM stereo with CD/DVD/MP3 player; Porsche Communication Management system with seven-inch touchscreen, navigation system and trip computer; auto-dimming outside and rearview mirrors; sunroof; automatic bi-xenon headlights; adaptive rear spoiler; Porsche Active Suspension Management adjustable suspension; and P245/50ZR18 front and P275/45ZR18 summer performance tires on alloy wheels.The 4S adds all-wheel drive with an automatic brake differential.

Panamera Turbo ($132,800) upgrades with adaptive air suspension with load-leveling and adjustable ride height, adaptive headlights, 14-way power seats with memory, power tilt/telescoping steering column, alcantara roof liner, full leather upholstery and interior trim, and P255/45ZR19 front and P285/40ZR19 rear summer tires.

Options are numerous, and you can get just about every part of the interior trimmed in leather, alcantara, wood or aluminum on an a la cart basis.Major exterior options include 19- and 20-inch wheels in several designs ($1,560-$3,380), nine colors and five special metallic colors, thermal and noise-insulated glass ($1,120), keyless access and starting ($1,090), roof rack ($400), and a rear wiper ($400).Performance options include the adaptive air suspension ($1,990); Porsche Dynamic Chassis Control with rear differential lock ($4,460); variable assist power steering ($270); Sport Chrono Plus package with analog and digital stopwatch, Sport Plus button and launch control ($1,320), ceramic composite brakes ($8,150); sport exhaust system ($2,950) with modified silencers, special tailpipes and an interior button to change the exhaust note.Front park assist and a rearview camera are bundled ($1,255).

Options inside include a full-leather interior ($3,655) with memory for the driver's seat and driver's side mirror; full-leather interior with 14-way power seats ($3,655) with power steering column and memory for lights, wipers, A/C, door locks, instrument cluster and PCM settings; and a full-leather interior with 18-way power sport seats ($5,360) that includes the same additional memory functions.The 14-way ($1,705) and 18-way ($3,210) seats are also offered without full leather.Also offered are front park assist ($600); adaptive cruise control ($2,490); large rear center console ($2,300); eight-way power rear seats ($1,835); front and rear seat ventilation ($1,600); four-zone automatic climate control ($1020); heated steering wheel ($210); power sunscreens for rear side windows ($450) and a power rear sunscreen ($340).Audiophiles will appreciate the 14-speaker, 585-watt Base surround sound audio system ($1,440) and the 16-speaker, 1000-watt Burmester surround-sound audio system ($5,690).Other audio options consist of XM satellite radio ($750), Bluetooth cell phone connectivity ($695), 6CD/DVD changer ($650), and a Universal Audio Interface ($440) with auxiliary input and USB jacks.

Safety features that come standard include dual front airbags, dual front knee airbags, front- and rear-side airbags, curtain side airbags with rollover deployment, tire-pressure monitor, rear park assist, anti-lock brakes, traction control, and electronic stability control.

The Panamera also had to look like a Porsche, and that means it needed elements of the 911.The 911 influences include the signature shoulders or haunches around the rear wheels, a hood that sits lower than the front fenders, a front end with lower air intakes but no grille and the rounded rear end.The sensor for the available active cruise control degrades the appearance of the car from the front.Hidden at the back of the car is a cleverly designed active rear spoiler.It rests under a chrome trim strip and pops up at speed to increase rear downforce.

The result is a car that looks awkward from some angles.The length added by the rear doors and the high rear roofline appear to stretch the car too far.It seems like it would look better if you could take about 18 inches out of the rear roof area and give the roof a sharper slope.But if Porsche did that, it would look a lot like a front-engine 911.The rear end looks bulbous, reminding us somewhat of the old 928.In short, we think styling is this car's weakest point.

The design may not be elegant, but the Panamera does have presence in traffic.It attracts attention when it pulls up to a luxury hotel or fine restaurant or other gathering.On the road, that large rear end stands out.

Panamera S and 4S models come standard with three partial leather upholstery choices, while the Turbo gets full-leather upholstery in four colors.Three different two-tone combinations and natural leather in two colors and one two-tone combination are also available.Interior trim consists of carbon, aluminum, and five real-wood options.And those who really want to personalize their vehicles can opt for an alcantara roofliner (standard on Turbo), extra leather on just about everything, including the rearview mirror, steering column, air vents, and the top of the dash.It's all very handsome.

The center console is replete with buttons, upwards of 32 of them.Other functions are controlled through the standard seven-inch touchscreen in the center of the dash and another 18 buttons surrounding that screen.Another 4.8-inch multi-function display is housed in one of the gauge pods in front of the drivers.It pairs with the navigation screen to show just about any information the driver might want.Porsche opted for a button for every possible command rather than a centralized controller along the lines of BMW's iDrive.We found all those buttons overwhelming at first, but it became simpler as we became accustomed to them.The buttons are logically grouped by function and easy to reach.A central controller might look more elegant but it would be even harder to learn.In short, Porsche's system is easier to learn than BMW's iDrive.We found the navigation system hard to figure out.For example, three of us, veteran automotive journalists all, could not in an hour-long drive figure out how to switch from the bird's-eye view to map view.

Instruments are housed in five tubes, with the tachometer front and center in white with black numbers.The speedometer, marked in hard-to-read 25 mph increments, sits to the left of the tach, and the multi-function display is to the right.Both of these contrast with the tach, using black backgrounds and white characters.If you can't read the speedometer, that's OK, because a digital speed readout is provided at the bottom of the tach.Two smaller gauge pods flank the speedometer and multi-function display, creating the five-pod arrangement.These pods include readouts for the fuel gauge, water temperature and oil pressure and temperature.

Buyers can opt for three levels of audio systems.The base system, with 11 speakers and 235 watts of power is quite good.The optional Bose surround sound system, with 14-speakers and 585 watts, is loud and clear.It matches anything you'd find in most luxury cars.We found the 16-speaker, 1000-watt Burmester surround sound is as clear as any we've ever heard, and we've herd some good ones.

We found the front seats firm and comfortable.Hop in any seat and you'll notice that the full-length center console, which rises toward the dash, creates four distinct seating pods, each of which offers all the room and comfort the vast majority of passengers would ever need.This is one sports sedan that doesn't compromise rear seat room.The center console was inspired by the unit in the Porsche Carrera GT, and the seating position is similar to that of the 911.Supportive bucket seats can be found at all positions.The base seats have 8-way power adjustments in the S and 4S and 14-way adjustments in the Turbo.Those who really want to spoil themselves can choose the 18-way front sport seats and the 8-way adjustable rear seats.

Rear-seat headroom is especially impressive, and can accommodate occupants well over 6-feet tall.We found the rear seats comfortable, like buckets.The copious space front and rear would make the Panamera a fine chauffer-driven vehicle, though giving up the driver's seat wouldn't be easy.Rear seat heaters are available, with rear-seat climate control.

Rear visibility is limited.The angle of the rear window makes it look like a rather short slit from the driver's seat.Otherwise, the mirrors provide good coverage.

Storage up front includes a cupholder at the front of a shallow center storage console, the glovebox, and door map pockets.In back, there is another cupholder, a shallow storage tray, and a small storage cubby in the fold-down armrest.

Cargo space is quite massive.The hatchback design provides plenty of space for larger items.With the rear seats up, there is 15.6 cubic feet of space behind them, which is about as much space as a large sedan's trunk.Those rear seats fold almost flat with the touch of two finders to open up 44.2 cubic feet of cargo volume.That's enough room for a family of four and their luggage on a weekend trip.Four suitcases fit easily in this car.

That's probably due to the car's advanced engineering and extensive use of lightweight aluminum and magnesium for portions of the body structure.The engine uses a dry sump oiling system rather than a standard oil pan so it can sit low in the chassis, and Porsche has placed it as far back as possible.

The Panamera comes with two forms of adjustable suspension, a standard system with gas shocks, and a full air suspension in the Turbo.The suspension adjustments allow the Panamera to drive like a luxury car or a racetrack-ready sports sedan, and it always feels smaller than its considerable size (which slots between a BMW 5 Series and a 7 Series).Most adjustable suspensions are either too soft or too firm.That's not the case with the Panamera.The base suspension's Comfort mode provides a smooth but controlled ride, while the Sport setting makes the car react quicker without ruining the ride.The Turbo's adaptive air suspension adds a firmer Sport Plus mode that's tuned for driving on a track or twisty road.The air suspension can also lower the car one inch for better handling and raise it 0.78 inch to help the front end clear curbs.Porsche also offers Porsche Dynamic Chassis Control (PDCC), which comes with active anti-roll bars.To counteract body lean in turns, the system twists the roll bars to make them firmer.The system can also disconnect the roll bars to improve straight-line comfort on bumpy roads.Given all these controls, you can transform the Panamera from firm and race-track ready to smooth and refined with the touch of a couple of buttons.

We had the opportunity to test the Panamera's potential on the 14-turn, 4.1-mile Road America road course in Elkhart Lake, Wisconsin.Though large and heavy, the Panamera was at home on this long racetrack, with quick, communicative steering and a relatively flat attitude through turns (especially with PDCC).The Panamera's willingness to change direction and respond to driver inputs puts it in a league with the world's best sports sedans (such as the BMW M5).

Road America has a lot of long straights, and the Panamera's brakes weren't entirely up to that challenge, exhibiting a pulsation that may have indicated warped rotors.On the road, the brakes are more than enough to be perfectly capable.Drivers intending to drive their Panameras on a race track or tackle twisty mountain roads on a regular basis should consider opting for the expensive but impressive composite ceramic brakes.

The Panamera is very fast.The base V8 in the S and 4S models provides more power than anyone really needs.It offers willing response across at all rev ranges, starting with a burst and delivering plenty of passing punch.With rear-drive in the Panamera S, 0-60 mph takes just 5.2 seconds.The all-wheel-drive system in the 4S does a better job of putting the power down, cutting the time to 4.8 seconds, in spite of its additional weight.Add the Sport Chrono Plus package with its launch control feature and both 0-60 times are 0.2 seconds quicker.

The 500-horsepower 4.8-liter V8 in the Turbo is brutally fast, knocking the 0-60 time down to 3.6 seconds with launch control.Kick the throttle and the power knocks you back in your seat, not letting up until you do, or 188 mph, whichever comes first, though we didn't check this last feature.Thanks to standard direct injection, turbo lag is minimal, if at all existent.Sure, the Turbo is overkill, but we like it.

The seven-speed PDK automated manual transmission works well as a smooth automatic if left in Drive, and becomes race-ready when the driver chooses the Sport or Sport Plus modes, which hold gears longer to make power more readily available.Those who want to shift manually, can tap the steering wheel buttons in any mode.We found that the Sport Plus mode chose the appropriate gear for track driving 95 percent of the time.

For all that power, the Panamera goes fairly easy on gas.It comes with a start/stop feature that imperceptibly turns the engine off at stoplights to conserve fuel.EPA fuel economy estimates are 16 mpg city/24 mpg highway for the S models and 15/23 mpg for the Turbo.No Panamera is subject to a Gas Guzzler tax.

Kirk Bell performed his test drive of the Panamera at Road America in Elkhart Lake, Wisconsin; NewCarTestDrive.com editor Mitch McCullough contributed to this report.