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I was going to save this for the end but now I’m here I don’t think I can keep it to myself. Porsche has failed with the Cayenne. By any of its own terms of reference, be it that the car should look, feel or drive like a Porsche, it has missed by a mile.

It was always going to. It might be able to make the Cayenne go fast and sound good, but 2 and a quarter tons of tall, front-engined off-roader was never and will never feel like 1 and a half tons of low-slung, rear-engined supercar. Not even Porsche can do that. Porsche’s gamble is that, to its customers, this won’t matter.

Two types of Cayenne will be sold in Britain from next April, the 350bhp Cayenne S (£44,530) and the 450bhp Cayenne Turbo (£68,970). About 2,500 units — 10% of total production — will find homes in the UK, four out of every five being the cheaper, normally aspirated S model.

The next thing you need to know is that, despite its failure to match Porsche’s lofty claims, this is the best 4x4 I’ve driven. Even the basic Cayenne S makes a BMW X5 look slow-witted, and every other competitor seems laughably sluggish and compromised. Only the Range Rover deserves to be mentioned in the same breath, and as its duties are firmly in the land of luxury while the Cayenne’s brief is entirely sporting, further comparison is meaningless.

But the Cayenne’s most extraordinary talent is also its most pointless. Far from creating a silhouette 4x4 that’s all Porsche sports car underneath, Porsche has created one of the great off-roaders. I know this because I have spent some considerable time in one teetering at insane angles on impossibly steep and slippery slopes listening to the whirrings of its traction control systems as they found grip where all logic told you there should be none. Even when I finally ground to a halt halfway up a 40-degree slope and apparently irretrievably stuck, within a second it had analysed the situation, delicately fed varying and appropriate amounts of power to each wheel and resumed normal progress.

I don’t doubt a Range Rover would have followed it but I seriously question whether an X5, which lacks even low ratio, would have got up the first slope.

It’s pretty startling on the road too. Even the S feels quick and, with a 0 to 62mph time of 7.2sec and a top speed of 150mph, so it should. Its brand new V8 motor sounds fabulous and, running through a six-speed automatic transmission, progress would be silken were it not for ride quality that is, frankly, pretty poor.

The flip side is astonishing handling for one so high and heavy. No matter that it would not trouble a Honda Civic Type-R or any car to wear a Porsche badge in the past decade down a twisting road: compared with every other 4x4 it brings a new level of grip, precision, response and fun.

The Turbo breaks still more ground for the category. It reaches 62mph in 5.6sec and will hit 165mph given sufficient space — it’s a superb engine, too, less aurally satisfying than the S but almost entirely lacking in lag and capable of providing quite brutal acceleration up to around 130mph. Its fuel consumption, however, is horrendous: ignore the official figures and be advised that any attempt to drive it enthusiastically will burn a gallon of unleaded in comfortably less than 10 miles.

But perhaps the biggest issue is whether Porsche should have built the Cayenne at all. I have no doubt that it will sell and that the Americans — who will buy more than everyone else combined — will lap it up in particular.

But what it does to the future perception of Porsche is another matter. On one level it is the world’s best 4x4, on another it is the cynical exploitation of a glorious brand that risks long-term damage to that brand’s very identity in the pursuit of easy money.

For more than 50 years Porsche has developed a stellar reputation building coupés and roadsters and now, from nowhere, comes a five-door sky-high off-roader. How would we feel, I wonder, if Land Rover tried to make a Porsche? Part of me says the Cayenne is a unique proposition and you only have to feel how much better to drive it is than a BMW X5 to know how good it is. But the remainder still knows that this car neither looks nor behaves as a Porsche should. I also believe that sooner or later the world will wake up to the profligacy of 4x4s and realise that gas-guzzling, CO2-belching, two-ton off-roaders with inevitably compromised on-road behaviour represent no kind of future at all.

So I applaud the extraordinary Cayenne and wish it every success while at the same time still wishing, in part at least, that it had never been built.

Vital statistics

Model Porsche Cayenne Turbo
Engine V8, 4511cc, twin-turbocharged
Power 450bhp @ 6000rpm
Torque 457lb ft @ 2250-4750rpm
Transmission Six-speed automatic
Suspension (front) double wishbones, air springs; (rear) independent, multi-link, air springs
Tyres 255/55 ZR 18
Fuel 17.9mpg (combined)
CO2 378g/km
Insurance Group 20
Company car tax Figures available early 2003
Acceleration 0 to 62mph: 5.6sec
Top speed 165mph
Price £68,970
Verdict Great car, if only it wasn't a Porsche