BMW's three-litre turbo diesel engine is widely regarded as being one of the finest units of its type in the world, and you won't find me arguing too strenuously against that opinion. It adds greatly to the quality of every car to which it is fitted, even including the long-wheelbase 7-Series tested here. This is a very large machine, and it's a credit to the diesel unit that it can push such a leviathan forwards with apparently minimal effort.It's also by far the best part of the car. During my time with the 730Ld I could hardly stop thinking about the long-wheelbase Jaguar XJ which I drove earlier this year and which impressed me so deeply. The engines of the two cars are so different as to be not worth serious comparison - either you want a petrol motor or you want a diesel one - but the space available for rear passengers is hugely impressive in each case.In all other respects, though, I could find no reason why anyone would choose the BMW over the Jaguar. The XJ seemed to have been designed round me; the BMW seemed to want me to redesign myself round it.An obvious example supporting this view is the notorious iDrive system which caused gasps of horror when it first appeared in the 7-Series and had to be toned down as it filtered through to first the 5-Series and then the 3-Series.The important thing with iDrive is not to show fear. It does its job effectively enough, but I feel that it can be used most easily by the people who created it, and that the people they created it for (that's us) were not considered to the extent they should have been.The stalks behind the steering wheel are not particularly user-friendly either. There are no fewer than four of them, all short and stubby, and one, rather confusingly, devoted to gear selection. Somehow I couldn't get used to controlling such a major operation by using such a tiny lever.I dare say I could get used to this and many other initially upsetting aspects of the 7-Series, but I don't see why I should have to. I get the impression that the car is challenging me to be worthy of it, and I want to remind it of who exactly is in charge here. It's like being poorly served by a surly waiter in an expensive restaurant, when you want to say something like, "If you're so clever, why am I the one who's sitting down and giving the order and you're the one who's standing up and taking it?"Similarly, if this 7-Series thinks itself so fine, why can't it cope with up-and-down and side-to-side motions as well as the turbo diesel allows it to deal with going forwards? The 730Ld doesn't like bumps at all, won't have any piece of them, refuses to consider their effect on occupant comfort and delete them from the driving experience. Its great size is all too apparent on corners, which would be understandable but for the fantastic handling ability of the Jaguar XJ. And, worst of all, the smallest inconsistency in the road surface sets up a rocking motion which at one point made me fear for a repeat view of my lunch.What, then, is all this about a BMW being "the ultimate driving machine"?There's an inconsistency about what BMW is doing these days. I've said before that, at the opposite end of the scale, the turbo diesel version of the 1-Series is much more enjoyable to drive than the high-performance 130i M Sport. You can find another example by comparing the 730Ld with a car which shares its wonderful engine - if you want a BMW with a brilliant turbo diesel under the bonnet and a chassis which combines an excellent driving experience with ride comfort that surpasses what can be found here, the car you should really be looking at is a 3-Series. Engine 2993 cc, 6 cylinders Power 231 bhp @4000 rpm Torque 384 ib/ft @2000 rpm Transmission 6 speed semi-auto Fuel/CO2 34.4 mpg / 216 g/km Acceleration 0-62mph: 7.9sec Top speed 148 mph Price From £50536.00 approx Release date 12/09/2005