Good: glorious cabin and comfort, sharp to drive in Sport mode, quality, refinement, that engine

Not so good: ugly, kind of pointless

There is a level on which I do really rather love the new BMW 6 Series GT. For a kick-off, I've always had something of a soft spot for big, pricey, comfy executive cars that, instead of a three-box silhouette with a boot, come with a giant sloping fastback. The Rover SD1, for example. The Citroen CX and XM. The Saab 9000. So I do, kind of, sort of, get what BMW is going for with the 6 Series GT.

But it is rather pointless.

Let's start with precisely what it is, which is... er... it's a... ummm... sorry, no I give up. It's called a 6 Series, but it's actually built on 5 Series mechanicals, has the cabin (more or less) from a 7 Series and boot space to rival a BMW X5. All of which should be brilliant, right? If you're taking the chassis of the great-to-drive 5, the cabin of the sybaritic 7 and the space of the hulking X5, and putting it all into one car, then it should be great, shouldn't it?

Is now a good time or a bad time to remind you of the episode of Friends where Rachel got her recipes confused and made a trifle that included components of a shepherd's pie?

Actually, the 6 GT isn't that bad, but it does seem to be rather sitting out on a limb of the BMW model range, or the car world in general. It's seemingly a car designed solely to be bought by airport chauffeur companies - a job that it would carry out with aplomb, but hardly calculated to appeal to the masses.

Compared to the old 5 Series GT (the car that, despite the numerical name change, it replaces), it's 87mm longer, 21mm lower and around 150kg lighter. It remains rather unattractive at which to look, though. The 5 Series 'face' looks handsome enough, and there's no denying that the rear end is a little sleeker (and 64mm lower) than that of the old 5 GT, but the middle section still looks bloated and un-sleek.

There is a payoff for that lack of sleekness, though, and it's the cabin. Which is wonderful, and I don't use the world lightly. The interior lifts all its components and parts from the 5 Series and 7 Series, and while that means it's all a bit generic-BMW parts-bin in there, every single component seems hewn from solid, and the levels of quality and assembly are just off-the-chart good. It's not as pretty as what you get inside an Audi, nor quite as impressive as the cabins of the latest Mercedes, but it is a fantastic space in which to spend time.

And there is a great deal of space. The 7 Series-like cabin dimensions mean that there is stretching room in the back seats for all but the tallest passengers. Our test car came with the rear seat comfort pack (part of an options bonanza stretching to more than €20,000!), which includes reclining seat backs, heated seats and sun blinds. It's one of those cars that, as you drive it, you start thinking 'I really wish someone else was driving and I was relaxing back there...'

Space and comfort levels in the front are barely any less impressive, and the technology count is high. You get digital dials, the new iDrive system (still the best infotainment setup around), a Harmon-Kardon stereo, a heads-up display and four-zone air conditioning. On the safety front, the 6 Series is bristling with sensors, and comes with active cruise control, lane-keeping steering, various collision warning and evasion systems and an automated parking system.

For those worrying that BMW has thrown the driving baby out with the driver aids bathwater, fret not; the 6 Series GT is good to drive. Note, though, that I say good, and not great. By BMW's standards, this big, comfy, outside-lane express doesn't enjoy being hustled on a tricky road as much as it might. It's not that it's especially heavy (it's around 75kg heavier than an equivalent 5 Series Touring estate), but that much of that weight is set higher up, so its reactions are a little slower, a touch gentler than that of the 5 Series. That's fine if all you want to do is cruise, and the 6 GT has exceptional levels of refinement and comfort, with a gentle ride quality even on the optional 20-inch wheels. Switch it into Sport mode and it definitely sharpens up a touch, and retains much of the traditional BMW feel and response, but you can tell that such hooliganism is not the kind of driving that the GT craves. It just wants to waft, and it's very good at doing so.

A true highlight of the 6 GT experience is that 3.0-litre straight-six diesel engine. BMW's six-pots are getting thinner on the ground these days, as smaller units get replaced with down-sized four-cylinder engines, but this one reminds you of why BMW has the reputation it has for making engines. Yes, it's diesel, so it's evil, but you won't care when you realise that you've got an engine that can beat a Golf GTI from 0-100km/h, returns 40mpg (if you're gentle) and costs €270 a year to tax. It's also very smooth and refined - a classic demonstration that, while turbos are nice and all, there's no replacement for displacement.

That does leave us with the question of quite what the point of the 6 GT really is. You can have all its quality, much of its space and that wonderful six-cylinder engine in a 5 Series Touring, which is handsomer by far, better to drive and more affordable. If you really, really want the space and luxury, then you'd probably be better off with a year-old 7 Series, because by the time you've specced a 6 GT up to the levels of our test car, you're past the €100k mark, and staring down the barrel of some very steep depreciation.

In so many ways, it's a very likeable, hugely capable car and if you've got a long, endless motorway journey to do, then I can think of few better cars in which to do it. But for the rest of the time, a 5 Series is a better choice.