Not as dramatic to look at as an X6, less impressive to drive than a 5-series – yet the 5-Series Gran Turismo still has more than enough appeal to face down its diverse opposition. A spacious cabin and practical boot helps, even if some developments such as the split tailgate and folding rear seats feel a bit gimmicky.

It’s the ultimate challenge for any new car designer. Build a machine that has no natural rivals, breaks fresh ground in design and engineering and sets buyers imagination on fire…

It all sounds straightforward enough, yet the huge complexity of the new car market means that there is a problem…If you can think of it, the chances are, someone else is already building and selling it.

Not so the new 5-Series Gran Turismo says BMW. By distilling the very best of the firm’s most popular models, including coupe, estate and four-wheel drive SUVs, the firm has created the first truly mold-breaking vehicle of 2009.

And when you see the car in the flesh for the first time, you can see why. With its split rear tailgate, arcing roofline and impressive dimensions, it certainly stands out as something decidedly new – even if it is fair to say the styling won’t appeal to everyone. From some angles, the car keeps the muscular stance BMW is celebrated for, from other angles it looks less purposeful.

However, there’s no doubting that it passes the test when it comes to choosing potential rivals. With such diverse influences, it really is hard to choose a car that really can stand toe-to-toe with the newcomer.

Based on the same chassis that underpins both the 5-series and 7-Series and powered by a family of advanced engines, including 3.0-litre diesel and petrol units and a flagship 4.4-litre petrol V8, the 5-Series GT boasts a large cabin and performance to match.

Our 530d test car was trimmed with comfortable leather seats, and offered every conceivable luxury. Chairs in the back can also be slid fore and aft, and can be reclined for comfort. Options include rear TV screens that offer internet access and control of the car’s navigation and entertainment functions as well as conventional broadcast content.

Opening the boot is simple. One button lifts the hatch in its entirety, while another, fitted alongside pops the tailgate like a conventional saloon. In fairness, while the hatch is huge, we felt the smaller opening accessed through the tailgate was too small to be of much use on a regular basis.

Fire the six-cylinder diesel engine, and there is more to admire. The engine really is incredibly smooth and at idle remains almost silent. Connected to an eight-speed automatic gearbox, the 240bhp unit delivers seamless in-gear performance, and a 0-62mph sprint of 6.9 seconds, while still returning an impressive 43.5mpg.

But there are further surprises. BMW’s Dynamic Drive Control offers three chassis settings including comfort, sport and sport plus. In comfort, the ride is almost magic carpet smooth – and to our mind, a step change in terms of its set up for BMW. The car puts refinement and smoothness at the core of its character - even in sport and sport plus modes.

On a winding road, the car remains involving to drive, though we would say there’s less steering feel than we have come to expect – and less immediacy of response from the controls. BMW’s argument is that this evolution is very deliberate – the firm aims to build a rounded, mature model in the 5-Series GT, creating a new benchmark for disparate rivals based on more than outright performance.

The good news is that despite this shift of focus, the Gran Turismo remains a mostly rewarding car to drive. But will families really take to the car in the way BMW hopes? With UK sales volumes expected to reach 2,000 units a year, the expectation is being carefully managed. However, there’s no doubt that what this car represents is crucial to the firm’s future – and from that perspective, it’s hard to underplay just how important each on those sales actually is.