A week behind the wheel of the most expensive C6 did nothing to resolve a serious question I have about Citroen's luxury range, but apart from that little difficulty I must say I became very fond of the car. As well as being a very effective mile-eater on motorways, it also proved to be unexpectedly capable in more demanding road conditions, to the point where I found hustling it through corners nearly as rewarding as wafting along the straights.
Citroen gives C6 buyers a choice of two engines. One is an excellent three-litre petrol V6; the test car had the alternative unit, the 2.7-litre turbo diesel co-developed by PSA Peugeot Citroen and Ford, and found in such otherwise unrelated models as the Peugeot 407, the Jaguar XJ and the Range Rover Sport.
As I've lost no opportunity in saying before, this is quite some engine. It may not have quite the storming performance of, say, BMW's three-litre diesel, but it's a sturdy device none the less. It certainly has no difficulty in carrying the considerable bulk of the C6, and it's so quiet that you might struggle to believe that the car could possibly sound more subdued than it does unless you happened to drive the still more refined petrol version on the same day.
Having done just that, I would add that there isn't much to choose between them in terms of performance either. The most noticeable effect of swapping engines is the deterioration in ride quality when you switch from petrol to diesel. In particular, the HDi rocks slightly from side to side on uneven surfaces which cause the petrol car no apparent distress whatever.
This is bad news for a luxury car, and it's the only area in which the HDi is inferior. If the ride quality had not been affected, I would have no hesitation in recommending the diesel as the better choice. As things stand - and despite the superbly relaxed characteristics of the HDi unit - it's difficult to justify spending an extra couple of thousand pounds on a less comfortable car.
Citroen has made a big point of emphasising the design connections between the C6 and the very few really large cars which appear further back in its history. The most noticeable of these is the imbalance between the very long front overhang and the curtailed rear, which was such a characteristic feature of the 1970s CX.
Not everyone likes it. One particularly critical onlooker suggested that two designers, neither of them aware of the other's existence, devised one half of the car each, and that the results of their labours were jammed together at the last minute.
I think this is unfair. The C6 may be unconventional (and what sort of place would the world be without unconventional Citroens in it?), but there's a certain elegance to it. And it will always be distinctive, partly because nothing else looks quite like it, and partly because - as Citroen itself admits - no more than a few hundred examples are likely to find buyers each year in the UK.
The exterior lines conceal a quite enormous cabin. Front occupants have a vast amount of space, but if anything they are given second-class treatment to those in the rear. Other than special long-wheelbase versions of cars produced by other manufacturers - the BMW 7-Series and Jaguar XJ come immediately to mind - I can't offhand think of any comparable vehicle with quite so much space in the back.
This is even more the case in the range-topping Executive model. It's the only one with the option of a £1000 Lounge Pack, which consists of electronically controlled rear seats designed in homage to those found in the French TGV high-speed trains. And they're very impressive, but they also lead to the question that has been bothering me about the C6: who exactly would buy one?
Badge snobbery suggests that hardly anyone with just under £40,000 to spend on a car would hand over a cheque of that size to a Citroen dealer, no matter how wonderful a vehicle they receive in return. The TGV rear seats imply a desire to be driven rather than to drive, yet how many people employ chauffeurs these days? Some do, of course, but who among them spends that sort of money on their first car rather than, say, their fifth?
Chauffeur hire companies might be a different story. I have a friend who runs just such a thing, and his car of choice is a Chrysler Grand Voyager. The C6 might be an interesting alternative for him to offer his customers. But you can't build a business model on selling a car to chauffeur hire companies and no one else.
The truth is almost certainly that Citroen has created the C6 and Citroen UK is obliged to make it available, knowing all the while that it will attract very few takers. And I'm increasingly of the opinion that that's a shame, because there is a lot to be said for the C6.
It is, after all, by far the best big car that any French manufacturer has come up with in the modern era. It's also one of the safest cars on the market (scoring phenomenal marks in the Euro NCAP crash test programme) and one of the most likely to foil any attempts to steal it (five stars in the "theft of" and "theft from" categories in the British Insurance Car Security Awards).
And apart from these and other attributes, it's simply a nice car to live with, and one I already miss just a few hours after I handed it back.