What exactly is the 2012 Mercedes-Benz R-Class? To be frank, we'd like to tell you, but even a simple definition would be tough to provide. We could call it a minivan, but it has significantly less interior passenger volume and no sliding doors. We could call it a wagon, but it's really too big and bulbous for that. Mercedes calls it a crossover, which is probably the best option given the lack of a better term, but the R is hardly consistent with the countless other SUV-like crossovers out there.

Regardless of what name you hang around its neck, the R-Class seats six people — seven, if you opt for the second-row bench seat. It might not be as spacious as a minivan, but it has more cargo capacity than virtually every luxury crossover. It comes with a pair of relatively efficient V6 engines -- one diesel, the other a new-for-2012 gasoline V6 -- and goes down the road with the sort of poise for which Mercedes-Benz is widely known. Inside the cabin you get a generous amount of equipment and the expected luxurious ambience of a vehicle carrying the three-pointed star.

All this sounds good, but there are significant drawbacks. The R-Class might travel down the road as comfortably as a car, but the road had better be wide because this car can feel absolutely enormous from behind the wheel. The R-Class is also pretty enormous in weight, and there's only so much the two V6s can do since more powerful V8 engines are no longer available. Those conventional side doors might look cooler than a minivan's sliders, but you may not care about aesthetics when your kid accidentally bangs one of the R's big rear doors into a neighboring car. Finally, the R-Class is a victim of its age, as it features an older, less user-friendly electronics interface than its newer Mercedes-Benz siblings.

Inevitably, families searching for a large six- or seven-passenger vehicle would be wise to check out more practical, less cumbersome and vastly less expensive minivans like the Nissan Quest and Toyota Sienna, or crossovers like the Buick Enclave and Ford Flex. However, such pedestrian name plates obviously don't carry the prestige of Mercedes-Benz, nor are they built to the same exacting standards. This leaves the 2012 Mercedes-Benz R-Class as a pretty sensible family mobile for luxury shoppers compared to large SUVs like the Infiniti QX56, Land Rover LR4 and the Mercedes GL-Class -- even if you can't really define what it is.

The 2012 Mercedes-Benz R-Class is a large crossover/wagon/van vehicle with a standard six-person capacity. An optional second-row bench seat ups the passenger count to seven. There are two trim levels, R350 Bluetec 4Matic and R350 4Matic, but the difference between the two is restricted to engine and tires.

The R-Class comes standard with 19-inch wheels, automatic headlights, LED running lamps, automatic wipers, a sunroof, power third-row quarter windows, cruise control, dual-zone automatic climate control, an eight-way power driver seat (with four-way lumbar adjustment), a tilt-and-telescoping steering wheel, MB-Tex premium vinyl upholstery, Bluetooth phone connectivity, an older version of Mercedes' COMAND electronics interface, and an eight-speaker sound system with a six-CD changer, an auxiliary audio jack, satellite radio and HD radio. The Bluetec gets run-flat tires.

The Premium 1 package adds a rearview camera, a power liftgate, power-folding mirrors, auto-dimming exterior and interior mirrors, a power tilt-and-telescoping steering wheel, driver memory functions and an iPod/USB audio interface. The Premium 2 package includes all of the above along with keyless ignition/entry (available separately) and an 11-speaker Harman Kardon surround-sound audio system. The Lighting package adds bi-xenon headlamps with a self-cleaning system. The Sport Appearance package (not available on the Bluetec) adds 20-inch AMG wheels and blue-tinted glass.

Stand-alone options include parking sensors, adaptive suspension dampers, a blind-spot warning system, a panoramic sunroof, heated front seats, a heated steering wheel, three-zone climate control, power rear sunshades, the Mercedes emergency communications system and a rear-seat entertainment system.

The 2012 Mercedes-Benz R350 4Matic comes with a 3.5-liter gasoline-powered V6 that produces 302 horsepower and 273 pound-feet of torque. A seven-speed automatic transmission and "4Matic" all-wheel drive is standard. Mercedes estimates the R350 will go from zero to 60 mph in a rather sluggish 8 seconds. EPA-estimated fuel economy is 16 mpg city/21 mpg highway and 18 mpg combined.

The R350 Bluetec 4Matic comes with a turbocharged 3.0-liter V6 diesel that produces 210 hp and 400 lb-ft of torque. It also gets a seven-speed automatic and all-wheel drive. Mercedes estimates a 0-60 time of 8.2 seconds. EPA fuel economy stands at 18/23/20 -- good but not really that much better than the cheaper gasoline-powered V6.

Every 2012 Mercedes-Benz R-Class comes standard with antilock brakes, traction and stability control, active front head restraints, front seat side airbags and full-length side curtain airbags. Also standard is PreSafe, which readies the seatbelts and can close windows and the sunroof if it detects an imminent collision. The Mercedes mbrace emergency communications system and a blind-spot warning system are optional.

In crash testing by the Insurance Institute for Highway Safety, the R-Class achieved the best rating of "Good" in the frontal-offset and side-impact tests.

The 2012 Mercedes-Benz R-Class is big and feels it, but its ride quality is comfortable and its high-speed stability is exemplary. Throw in appropriately subdued noise levels and you've got all the ingredients of a world-class long-distance cruiser. However, maneuvering the R-Class on tight streets and in parking lots can be a chore because of its size, so the optional parking sensors and rearview camera are certainly recommended.

As for engine choice, the new-for-2012 gasoline V6 is more powerful and efficient than before, so the diesel-powered and slightly pricier Bluetec is no longer the easy choice it once was. Still, we'd probably go for it since the diesel engine is stronger at low speeds around town, with better fuel economy and only minimal cost increase.

The cabin of the R-Class certainly warrants its Mercedes badge, with high-quality construction and materials. However, the cabin controls are dominated by Benz's previous-generation COMAND electronics interface, which features a less convenient four-way directional button pad (like an old video game controller) rather than the new multipurpose knob found in the newer GLK-Class and other Benzes.

There are other downsides. The sliding doors of a minivan might not be cool, but the elongated conventional doors of the R-Class (especially the rear ones) can be cumbersome in parking lots. And while the six-person configuration offers plenty of room for all, the optional seven-passenger bench encroaches upon third-row knee room. With the second- and third-row seats folded forward, the R-Class can hold 85 cubic feet of cargo -- less than a minivan, but on par with large luxury crossovers.