An esoteric maxim that guides test pilots is: “You don't know what you don't know.” A version of this may now have been adopted by BMW. The company is introducing a car, which, it says, fits a niche in the market that did not exist. So how could a car have been designed for it if it wasn't there? Does BMW really mean that it has created some extraordinary machine the like of which has never before graced a road?

Apparently. “A completely new type of vehicle, a unique design concept and driving dynamics never seen before - all this becomes reality in the new BMW X6,” is the flowery prose that opens the official description of BMW's latest product. For some reason, this advanced engineering-led company that consistently reaches very high levels of quality and driving capability has to resort to this desperate and unnecessary hyperbole.

So it is best to disregard this stuff and just take the X6, described as the world's first Sports Activity Coupé (SAC), for what it is: a huge and definitely not pretty, two tonne-plus, highly technologically competent four-seater that has been styled to separate it from more practical but less visually memorable SUVs (Sports Utility Vehicles) by having a roof that slopes steeply to the rear. The bit about “driving dynamics never seen before” refers to its very impressive, standard-fit Dynamic Performance Control that can increase power and torque to the outside wheel in a slippery bend to help the car remain securely on its line.

Clever stuff, indeed, but do I recall something like this first being fitted to tanks decades ago? Well, I must be wrong because it is “unique the world over”, trumpets BMW.

I suspect, though, that such nit-picking is irrelevant to potential buyers of the car, because it represents wheeled image par excellence.

So, having considered all this, I climbed into the SAC (which costs between £41,965 and about £50,000), started its mighty V8 4.4-litre 402bhp twin-turbo engine and set out from Greenville, South Carolina, close to the plant where the car is built, towards the Blue Ridge Mountains.

For those with a CO2 conscience, there is also a choice of a six-cylinder 3.0-litre petrol and two very fine and sensibly efficient 3.0-litre diesels that will be the big sellers in the UK.

And then I began to realise what the X6, with its bullyboy looks, is about. It is a delight to drive; its weight and bulk are forgotten, it drives “small”, picking its way quickly and precisely along slippery, narrow, precipitous, winding roads, the V8 growling grizzly-like when roused. Its optional “active” steering is fine, its brakes do deserve superlatives, its driving position is commanding, and the miles simply fade away beneath its 20in wheels.

This all-wheel-drive bruiser of a cruiser is slightly longer and wider than the X5 on which it is based, with almost all the creature comforts that could be needed, although I found its gearshift paddles clumsy. BMW, with all the tenacity of a tungsten-toothed rottweiler, still clings to its irritating iDrive control system.

Despite the plunging roofline, I was comfortable in the back of the BMW, which has two, separate rear seats; great for keeping squabbling kids apart. Some dogs will look askance at the high boot lip and probably request power assistance or Zebedee springs to board.

On a test track, the X6 behaved magnificently, barrelling into corners at three figure speeds, its chassis electronics - they should be members of an automotive MENSA - sorting out my ineptitude before the V8 (with CO2 emissions of

299g/km and fuel consumption figures on the combined cycle 22.6mpg, 0-62mph 5.4sec) climbed to its red line again. Amazing.

The look of the X6 initially shocks you - particularly at the rear - but, after a couple of days, I found myself cautiously admiring many aspects of this massive, light-blocking, mechanically competent, high-quality machine with only four seats, that is thoroughly at variance with commonsense UK transport solutions, even with an efficient diesel engine.

BMW reckons to find about 1,600 buyers this year that didn't know what they didn't know - and what they've been missing. Until now. I wonder who they are?