You'd be forgiven for thinking that with the old-timer G-Class, the established M-Class and the new GL-Class, Mercedes-Benz would have had quite enough big SUV models in its showrooms to keep its customers satisfied. But no.Enter the six-seater, all-wheel-drive R-Class. Call it an upper-segment family, executive or estate car, an SUV, an MPV or any combination of the above and you wouldn't be wrong as the R-Class is more things than it isn't. When first launched towards the end of 2005 Mercedes referred to it, not as a crossover, which it also is, but as a "Grand Sports Tourer". However, the current official description has more recently transformed into "Luxury Touring SUV".Confused? You will be, as in addition to the R 350, R 500 and R 320 CDI models already on sale, Mercedes is about to offer an entry-level R 280 CDI as well as a monster AMG R 63 version which boasts a whopping 503bhp 6.2-litre V8 and two pairs of chromed tailpipes, and runs on beach ball-sized (20" or 21") five-spoke alloys. Pigeonholing this beast is even harder as you could also throw the terms GT, Q car or "autobahn bruiser" into the mix.But whatever you want to call it, you certainly get a lot of motorcar for your money, even taking into account the R 63's hefty £74,115 price tag. For a start, the standard R-Class is BIG by any standards, but the AMG version will only be available in long-wheelbase guise with an extra 235mm over the 2980mm of the SWB model.So, standing at just under two metres wide, over 1.6 metres tall and over five metres long there's plenty of room inside for six (without much luggage); or, with the second and third row seats folded flat, a cavernous 2385 litres of loadspace becomes available.As you would expect, the spacious interior is suitably sumptuous and the long list of standard equipment on the R63 includes a seven-speed Speedshift auto, adjustable sports air suspension, adaptive damping, traction and stability control, water-cooled speed-sensitive power steering and even something called Racetimer which records lap times, fastest lap, lap distance and your average and top speeds while on a racing circuit - you don't get that on too many MPVs or SUVs.Also distinguishing the AMG R-Class from more humble versions are redesigned front and rear aprons, enlarged intakes, AMG door sill panels, stainless pedals, sportier seats and steering wheel and, on my German-spec test car, a red-needled speedo which reads to 320km/h (almost 200mph). Should your budget stretch to a little extra then the R 63's options list includes such treats as crash-responsive head restraints, multi-zone climate control, a panoramic sunshine roof, a reversing camera, surround sound and a trailer coupling.With a kerb weight of a little over 2.2 tonnes you might reasonably expect the R63 to be something of a tank to drive, and there's no denying that it's sheer physical size does indeed require extra care, especially in tight spaces, but AMG engineers have again done an impressive job.Although steering precision and response isn't on a par with that of a thoroughbred sports car, the R63 grips tenaciously and goes where it's pointed, though hustling it swiftly along twisting, single carriageway roads certainly requires both commitment and concentration, with a dollop of raw skill not being superfluous to requirements either.Thankfully the R 63's enormous disc brakes make light work of hauling it down from heady autobahn speeds, but more impressive is that the big, naturally-aspirated V8 not only bellows raucously though not intrusively - like a V8 should - but, in conjunction with that seven-speed transmission, it delivers surging, seemingly relentless acceleration right up to and doubtless well beyond the 135mph or so I found the space for on this test drive. (Top speed is electronically governed to 155mph.)Although weight and high performance do not make easy bedfellows, in this case the R63's tonnage, along with clever suspension and aerodynamics, helps the car feel very planted at high speed.There's no arguing that the AMG R 63 is a devilishly efficient and accomplished machine (even with frequent full throttle blasts on the autobahn the trip computer matched its official combined mpg figure of 17.3mpg) but at the same time it's hard not to think that this car - for all its power and speed, seating and load space, luxury, status and all-wheel drive - is the answer to a question which nobody asked, or even thought of asking.Furthermore, although the R-Class certainly has an unmistakable presence, especially in this hot-rod AMG guise, surely only its mother could call it pretty. That said, I couldn't help but like the brute. Engine 6208cc, 8 cylinders Power 503bhp Transmission 7-speed automatic Fuel 17.3mpg Acceleration 0-62mph: 5.1 seconds Top speed 155mph Price £74,115 Details correct at publication date