In a leather-and-chrome meeting room, windows thrown open to let in the mild autumn air, we wait for the arrival of Klaus-Gerhard Wolpert, Porsche's director of SUV Operations, and Harm Lagaay, Porsche's design studio chief. The atmosphere's a little apprehensive, the sense of unease intensified by the gritty ebb and flow of a flat-six being bench-tested: a timely acoustic reminder of what Porsche is really about.

It's to this evocative backbeat that Wolpert arrives. It's unusual to get undivided attention from such senior personnel, and although undoubtedly a privilege, I get the impression Porsche is wheeling out the big guns because it feels pressure to justify the Cayenne. And in a way it does. Not to shareholders, as Porsche remains a family-owned company, but to those people whose emotional stake in the company transcends the fiscal: staunch customers and enthusiasts like you and I. So, why is Porsche building the Cayenne?

Money is the simple, if predictable, answer. The success of Cayenne, or the Third Line to use its imperious in-house moniker, guarantees the continued independence and security of the company, and in turn generates more revenue to invest in cars like the 911, Boxster and Carrera GT. You can't argue with the logic, but as Porsche is already one of the most profitable companies in the world (pre-tax profits up 40 per cent year-on-year to a mind-boggling ΂£500million), cash clearly isn't a problem. Perhaps, buoyed by such rude financial health, Porsche simply wants to take the opportunity to torment BMW, Mercedes and Ford in a sector previously thought to be beyond the reach of a small, relatively low-volume high performance car manufacturer...

Whatever the motive, when the decision to build an SUV was taken, back in 1998, Wolpert and his team were presented with the prickly engineering paradox of taking one shot at two targets. Together the X5 and Range Rover have the SUV market covered, but as neither combines peerless on- and off-road performance, they sit at opposite ends of the SUV spectrum. Only by tackling both head-on could the Cayenne stay true to Porsche's high standards and engineering integrity. No surprise then that Porsche has gone about building an SUV in its own meticulous way. According to Wolpert there's been no engineering compromise, despite sharing the development programme with VW, nor has Porsche had to dilute its core values for the Cayenne. Given the company's engineering track record, it's probably safe to take his word on the former claim, but can an SUV that weighs comfortably in excess of two tons really honour Porsche's obsession with driver-focussed, rewarding dynamics? We'll find out soon enough.

Wolpert makes a point of emphasising the 'Sport' in Sport Utility Vehicle, and with good reason judging by the technical specification of the range-topping Turbo we're here to drive. Thanks to an all-new, 4.5-litre, bi-turbo V8, headline power and torque figures shame many a supercar: 450bhp at 6000rpm backed up by 459lb ft from as little as 2250rpm, to be precise. Enough to power the Cayenne Turbo to 62mph in 5.6sec and on to an astonishing top speed of 165mph.

Channelling this prodigious output to the tarmac (and dirt) is an equally all-new six-speed Tiptronic automatic transmission and Porsche Traction Management (PTM) four-wheel drive system. Just as in the Boxster or 911, the Tiptronic infinitely adjusts its shift points to suit the individual driver's style, but now there's also a hill recognition mode whereby the transmission holds onto a low gear on long uphill and downhill gradients.

In normal grip-at-all-four-wheels conditions PTM settles on a front-to-rear torque split of 38:62, giving the Cayenne a natural rear-biased balance. However, when challenged off-road, PTM has the capability to direct 100 per cent of the force to the front or rear axle. Sophisticated interaction between PTM's processors and those controlling the Porsche Stability Management system, ABS, ASR and ABD, ensures that the Cayenne is kept discreetly on track.

Standard on the Turbo and an option on the entry-level Cayenne S, air suspension is an engineer's dream, enabling the car to be lowered at speed, raised off-road and maintain ride height irrespective of load. With six preset ride heights, the range of adjustment is nearly 5in (125mm), with a maximum ground clearance of 10.75in (275mm).

With axle articulation to rival a Range Rover's and a forthcoming Advanced Offroad Technology Package that includes sill protectors, steel underfloor protection, fully controllable differential lock on the rear axle and anti-roll bars that can be disconnected at the flick of a switch to increase articulation even further, the Cayenne's all-terrain ability looks as formidable as its autobahn potential.

Having put the objective engineering case for Cayenne, Wolpert retreats, leaving the forum open for Harm Lagaay to discuss the subjective merits of the Cayenne's styling. He exudes the cool, calm assurance you'd expect of one of the world's most experienced car designers, but the Cayenne's look has had such a rough ride in the press you'd forgive him for being a little bit defensive. Fat chance.

'I don't mind if people say they aren't sure about the Cayenne,' he says in a display of unexpected humility. 'Porsche's identity isn't contained within the side profile of a 911, it's in the subtlety of surfacing, the way the light catches the side of the car: "bones" we call them at Porsche, meaning the way they reveal the bone-structure of the car. Design is about pushing and pulling, finding the balance between moving things on and going too far.'

It's a healthily candid attitude to have, especially as first impressions suggest the Cayenne is, to coin evo photographer Andy Morgan's phrase, a 'Marmite' car: you either love it or you hate it. It sounds like a cop-out but, after a day in its company, I think this sort of reaction is restricted to your first glimpse. You really need to see it in 3D, outside, in natural light. You need to walk around it, crouch down, get close, see it moving, even drive it, before you begin to understand why it looks the way it looks. Whether you then decide to like, dislike, love or hate Cayenne is up to you. Lagaay can live with your decision, so long as you've given it, and yourself, that chance.

One thing's for certain: it's unmistakably Porsche. This distinct identity has not been achieved simply by the application of a 'family' grille, lights and C-pillar profile, either. Of course, the nose carries headlights and intakes that contain heavy 911 Turbo influence: without a range of saloons and estates from which to plunder a set of visual reference points, how else could Porsche bestow an instant identity on the uncharacteristic Cayenne? Simply putting a Carrera on stilts and calling it Cayenne wasn't an option. Equally, placing a Porsche badge on a derivative SUV shape and hoping the brand would do the rest was equally abhorrent.

Subtle curves and sensuous surfaces aren't something you expect to find draped over a gallumphing, two-ton, 165mph SUV. Nor are they obvious attributes of the Cayenne when judged purely from photographs, but look close, run your hand across the metal and you'll find them: where pillar melds into rear haunch is a prime example. Lagaay is the first to admit the line between stimulation and alienation is a perilous path to tread, but it's his firm belief that a designer should strive to push people's perception and shake the status quo, albeit without deliberately courting controversy. That the Cayenne exists at all is controversial enough, and Lagaay must have been at pains not to intensify the shockwave. Time will tell whether he's achieved his aim.

The Cayenne's styling may take time to get comfortable with, but I doubt the sheer absurdity of its performance will ever wane. With a minder from the factory up front in a 911 Carrera we're cutting a swathe through the morning traffic, but it's only when the other drivers get a rear-view mirror full of charging Cayenne that the outside lane truly empties. Nothing, not even a bright orange Murci΃©lago, has more mirror presence. It doesn't just clear the road ahead, it evacuates it.

Almost an inevitable consequence of such brutal physicality and explosive performance is the fact that you become a complete tarmac tyrant, obsessed with bullying and cajoling yourself into enough space to let the Cayenne rip. And rip it does, to an easy 150mph, uphill and down dale, with plenty of urge in reserve. In Germany, Cayenne Turbo is a motorway monster. In the UK it's the end of your driving licence.

After comfortably cruising at two-and-a-half miles-a-minute, our chaperone steers us off the autobahn and towards some uncharacteristically entertaining German minor roads. It's drizzling and the roads are slick with leafy autumnal flotsam - a tough test for any quick car, let alone one with the power of a supercar, the bulk of a semi-detached house and a towering centre of gravity.

Perhaps because of this you initially treat the Cayenne like a handgun, pointing it and firing it down the straights. Soon though, tickling around the corners isn't enough. This is a Porsche after all. Brake later, drive deeper into the heart of the corner and the Cayenne responds, not with sulky understeer and clumsy body-roll, but with tidy turn-in, strong grip and tangible poise. Carry too much speed or get on the power too soon and the nose will begin to wash wide, but it does so modestly and progressively. Small inputs make a difference, whether they are via the steering wheel or the throttle, and you can adjust the balance and stance of the Cayenne mid-corner.

That's not to say it has the responses of a sports car. With two tons or more of momentum chasing you down the road, it's never going to feel as lithe and inertia-free as a 911. What's encouraging is that you can drive Cayenne, get into a flow and rhythm with it to the point where you can genuinely have some fun. The brakes, complete with house-brick-sized front callipers, never tire of slowing this most portly of Porsches, and with PSM perfectly dialled-in to the Cayenne's limits you can hustle it along at a sustained pace that feels ever so slightly perturbing. I doubt an X5 would hold onto it for long, although an Audi RS6 Avant would more than keep it honest.

And there's the rub, at least for enthusiasts like us. Supreme though the Cayenne is for an SUV, in pure driving terms it's easy meat for a similarly priced conventional high performance saloon or estate. But then Porsche knows that the kind of person who's going to buy a Cayenne already has an exotic or two in the garage, so ultimate on-road ability isn't paramount. Rather, they want a versatile family car that can cope with anything, while also possessing enough performance and involvement to keep them entertained.

All of which makes Cayenne a tough car to rate in terms of evoness. If you need to tow 3.5 tons across uninhabited, unmetalled terrain at 165mph and look good while doing it, the Cayenne is a five-star performer. In terms of pure driving thrills it's a three-and-a-half star experience, but when you factor-in its presence, image, quality and all-round desirability it earns a solid four-star rating. Though it likely pains the enthusiast in all of us, it would appear Porsche has pulled it off.