Good: staggeringly comfortable, Star Trek tech, good to drive, great engine.

Not so good: looks too plain, is it really worth all that money?

Just as we all salivate over the grudge matches of Arsenal versus Spurs, or Liverpool versus Manchester United, so too there are derby matches in the motoring world. Mustang versus Camaro for example. Ferrari versus Lamborghini. Tesla versus the Martians. Well, maybe that last one is a ways off yet, but Elon Musk will get us there, you wait and see.

And, of course, there's BMW 7 Series versus Mercedes-Benz S-Class. Actually, this is a match that doesn't go back all that far. It was 1977 before BMW actually got around to making a full-size luxury saloon to challenge the Mercedes 500SE and SEL and for a long time no-one in Mercedes would ever actually admit that the S-Class, or the company itself, actually had any rivals.

One suspects that Stuttgart won't make the same mistake this time... While the contest between these behemoths has hitherto settled down into a fairly regular pattern (the 7 Series being more fun to drive, the S-Class more overtly luxurious and generally considered the better car overall), I think the introduction of this, the G11 and G12 model 7 might just be enough to change the game a bit, and the 7 and S are now fighting on a much more level playing field.

And BMW has done this by ruining the 7 Series. Well, not ruining it exactly, that would be too harsh, but there is certainly a species of specific BMW fans that will arch their blue-and-white eyebrows and flap their driving-glove-clad hands at the prospect of what BMW has here wrought. 

We've been used, of course, to BMW (and car makers in general) adding more and more layers of technology to vehicles and the new 7 is no exception. There are layers and layers of sub-menus (which the updated iDrive system does, in fairness, make as easy as possible to navigate, although you do have to make an initial guess as to which category the thing you want to change comes under...), but the key trick here is found in the now-familiar button that allows you to switch between Comfort, Sport and Eco Pro modes. Keep it in Sport and you'll find that the 7 Series is much as it has ever been - surprisingly sharp and reactive to drive for such a big, hefty car (and remember that this is the long wheelbase model we are reviewing). The steering is a touch numb (not helped by the over-padded steering wheel), but while comfort levels are definitely improved, and closer by far to what Mercedes does with the S-Class, there is still a core of Ultimate Driving Machine in here. 

Switch to Comfort or Eco Pro though and you find the bit that will horrify BMW enthusiasts. There's what I can best describe as a sub-clause in the menus for these settings that takes the standard-fit air suspension and, apparently, replaces it with Citroen Hydragas units from the 1970s. Honestly, enter Comfort Plus mode and the 7 turns from sharp, responsive sports limo into mobile sofa. It is staggeringly comfortable and soothing, and even a bit soft and floppy in the corners. There's an underlying sense of precision and responsiveness, but it's tucked away under layers of mattress, like the pea far beneath Hans Christian Andersen's princess. It is the most shocking thing I've ever seen BMW do and it's wonderful - it turns the 7 into what it should have been all along: a lounge on wheels, a four-person first class cabin. Honestly, who wants to drive a limo like a hot hatch?

The thing is, you might still want to, so keep the Sport button close at hand. The 730d's engine, still a turbocharged straight-six diesel, now has power and torque that give it quite staggering performance. A figure of 265hp doesn't sound like a huge amount, but 620Nm of torque is pretty healthy and, better yet, the car is light. I don't just mean light for a limo, but really light. BMW has used some of the carbon-fibre technology that it developed for the electric i8 and i3 in the 7's structure, so with nobody on board, this long wheelbase model weighs just 1,830kg - lighter than an old-shape Jaguar XF. Thanks to the lack of mass, this massive saloon will beat a Golf GTI in a drag race to 100km/h, crack 250km/h at its top end if you have a handy Autobahn in your back garden and yet returns a pretty easy 40mpg in daily driving. And you'll pay just €280 a year to tax it. Whatever about the morality of giving plutocrats cut-rate tax discs, you've got to admit that's some pretty impressive engineering.

Not half as impressive as the comfort inside though. Quite apart from the magical Citroen-esque suspension, the seats are massive and squishy and lovely. You could happily live in this car if you could work out a way to fit a toilet and shower in the boot. Our test car came with the €7k Rear Seat Comfort package option, which includes reclining, heating and massaging options, plus a TV screen in both headrests that are controlled by a small Android-equipped tablet mounted in the centre rear armrest. Kept the kids very happy on a few long journeys, that did.

There's more tech too, including a surround-view parking camera system that uses clever software to make it appear as if a drone-mounted camera is hovering several feet behind either shoulder; and fully active LED high-beam headlights; and Gesture Control. Yes, you can control the stereo without touching it. Simply waggling your finger around in front of the central infotainment screen, Obi-Wan Kenobi style, will turn the volume up or down. To be honest, it's a bit of a gimmick and not actually any less labour-intensive than using the existing steering wheel controls or the knob on the radio panel, but expect to see it on all BMW models soon - it's only a €245 option on the 7 Series.

So is that what the 7 Series is now? A mobile branch of Currys with a massaging sofa in the back? Not quite no, but there is no doubt that BMW is hiding the car's light under a bushel. Stung by criticisms of the 2002 7 Series, the Chris Bangle-designed E65, BMW has kept rigidly to the Bigger-3-Series styling of old. Understandable, especially in such a conservative end of the marketplace, but when the same company makes the glorious-looking i8 and the sexy 6 Series Gran Coupé, it's hard not to be disappointed. 

There's also the question of whether the 7 Series is still actually worth the money - and that's a trickier one to answer. The BMW 5 Series, still at the top of its game in spite of being replaced later this year, is such a brilliant car now, and can be specced up with many of the same interior toys that you find in the 7. Technically and engineering-wise, a 5 is nowhere near as sophisticated, but in the right spec, the qualitative gap between the two cars is narrower than you'd think, making the 7's €100k price tag harder to justify. 

Still, it's an amazing technical achievement, mixing old-school 7 Series handling fun with S-Class matching comfort and quality. Does the S still have a faint edge? Possibly so, and it would take a back-to-back test to divine its width, but the 7 Series now runs the big Benz closer and closer than ever, and it may just have its nose fractionally in front right now.