This is not the usual new car drive report. Contrary to normal practice, there'll be a lot of talk about the boot and the back seats, because that's the only way for you to judge whether or not BMW really has invented a new kind of car. And if it has, whether it's one actually worth having.
But there's also our usual meat'n'potatoes, stuff about the engines, transmission, handling and ride - which are all borderline brilliant as it happens. Those things matter because while the 5-Series GT is in itself decidedly minority-interest, you're to all intents and purposes at the wheel of next year's 5-Series saloon and Touring.
Right. The boot. If a manufacturer goes to all the trouble of inventing a new kind of car, you'd think it would deploy all available language to explain the thing. Yet bizarrely BMW people refuse to call this a hatchback, even though that's basically what it is. Instead they call it a GT, which is one thing it isn't. A GT in my book is a short-wheelbase plush but lightweight coupe for comfortable high-speed touring. The 5-Series GT, on the other hand, is a five-door with a body and seats of such Heath Robinson complexity that the thing weighs two tonnes. Unsurprisingly, confusion reigns.
The reason BMW skirts around the H-word is that in BMW-world a hatch is a bit blue-collar. Serious German business people drive saloons, apparently. To explain the idea of the 5GT, BMW made a cartoon video in which our suited hero gives some work colleagues a lift to a business lunch meeting in a car that doesn't have a ‘fully separated luggage compartment'. His facial expression indicates it's just dawned on him that his next promotion will be very much in an unfortunate sideways direction.
So although the 5GT does have a hatch, you don't have to use it. Instead you can open a smaller conventional lid inset into the main hatch. There's also a complex rigid parcel shelf and a movable bulkhead between the boot and the back seats. This helps cut off noise from the back tyres and exhaust, and makes the back seats feel like a part of the car rather than a part of the boot. And protects our animated hero against career meltdown.
But that boot-lid is hopelessly tiny. If you want to put much at all in the 5GT, open the whole hatch. Here the problems really kick in. In order to engineer this lid-within-a-lid idea, BMW has ended up with a colossally heavy tailgate that takes huge effort (or a £500 optional pair of slow-winding electric struts) to open and shut. Oh, and the structure of the hatch - and the extra sets of hinges and struts within it - is so bulky that it eats into potential boot space and kyboshes rear vision. The park radar is standard for a very good reason.
But there's more to the 5GT's oddness than its bootlid. The body has been made higher than a 5-Series saloon or estate. I can see the attraction of this idea. The seats are a couple of inches higher (more in the case of the rear ones) which helps visibility and gives a flavour of the command driving position that SUV buyers love. It also improves legroom, while the raised roof does the same for headroom. But unlike an SUV, the floor, engine, suspension and other heavy bits aren't raised at all. So the thing still has the centre of gravity of a car. Which means - hallelujah - it should handle like a car.
To make best use of all this space, the rear seats recline through a comfy arc. If you don't need all the legroom, they also slide a bit, which lets you push the bulkhead forward and get more into the boot. They also fold like a conventional hatchback's, though they don't actually give a flat floor. Normal spec is called SE, but the £2,900 Executive pack means two electrically adjusted and heated individual rear seats instead of a manual three-across split bench.
So there's lots going on inside this car. This gave the exterior designers some issues - especially the height. Despite the use of various tricks to make the 5GT seem longer (reverse-raked front grilles), lower (fastback tail) and wider (big chrome strip across the rear) it looks from every angle like it hasn't just driven to business lunches but partaken in too many of them.
From the driving seat, the view takes a sharp turn for the better. For a start, although the dash architecture differs from the 7-Series', the main controls are shared. That means marvellously clear analogue dials set into graphically satisfying and informative digital backgrounds, and the new-gen iDrive which is a lot more usable than of old. Because you sit high, it's easy to place the car on a narrow road, though at junctions you curse the thick pillars.
The 550i has a twin-turbo V8 (I didn't drive it) and the 535i an all-new 3.0-litre turbo six of 306bhp. It now has one turbo instead of the old 535i's two, and Valvetronic has been added. Nice in theory, but the power delivery is a bit sticky and it doesn't sound as gorgeous as a BMW six ought to. So have the diesel, which is nearly as quiet and still cracks seven seconds for 0-62. All 5GTs have a new eight-speed auto as standard. Its wide ratio spread and clever friction-reduction mean it's more eco-friendly (a superb 173g/km) than a manual would be, they say. The shifts are smooth, and super-quick if you use the manual over-ride. It feels like a twin-clutch. Which is good.
The test cars had optional Dynamic Drive active anti-roll, which allows the car to be remarkably supple on the straights (and I mean remarkably supple, far better than a 7-Series), yet still beautifully resolved in corners. In fact, if you switch to the sport setting, the rear bar stiffens up more than the front one in tight corners to kill understeer. Even so, it never shakes off the feeling, in S-bends, that it's a two-tonne car. By the way, avoid the four-wheel active steering option: better manoeuvrability in car parks in exchange for much worse feel and precision on B-roads is a bad bargain.
The 5GT is 100kg more than a 7-Series. It could lose some of that if it had a simpler tailgate and back seats and it'd be just as useful and roomy. Still, it does show that next year's 5-Series ought to be a real winner.