The American journalist simply would not give up. At the press conference for the recent launch of the Mercedes-Benz CLS-class, our German hosts proudly extolled the virtues of what they insisted was a radical new coupé. “It’s not a coupé,” said the American, “it has four doors.”

There followed a long explanation from the Germans of the “thrilling symbiosis between elegant design and the functionality of a four-door saloon”. Then the American again: “But it’s got four doors . . .”

That is the central conceit of the CLS-class Mercedes. The Germans say it’s a coupé that happens to have four doors. Suspending disbelief, you can certainly argue that the CLS has the styling cues associated with a luxury coupé, such as shallow side-glass and a long, low profile. This car is 1½in lower than the more conventional E-class saloon and a full 3½in longer: so in the flesh the CLS is a dramatic car to behold.

But while the CLS is significantly longer than the E-class, none of that extra length is in the wheelbase — so how much of a compromise is the car’s cabin? To use the Germans’ own spin: if you believe the car is a coupé it seems quite spacious, but if you take the view that it’s a saloon you’ll find headroom and rear legroom somewhat off the pace.

And you won’t find any relief by specifying the full-length glass roof that other Mercs can have, because designers reckoned the sunblinds necessary for such a setup would rob too much of the car’s precious headroom. The CLS is also strictly a four-seater, as the individual rear seats are separated by a solid console running the length of the cabin.

However, compared with any coupé in its price range — likely to be £41,000 for the entry-level V6 and £50,000 for the top-spec V8 — the CLS does offer a spacious rear cabin: anyone less than 6ft tall will find it okay back there. It also feels like a well-built interior of reassuring quality — not something to be taken for granted with Mercedes these days.

When it goes on sale here next March the CLS will be available with a 272bhp V6 or a 306bhp V8. It’s a heavy old thing, though, weighing in at nearly 2 tons. So while the V6’s claimed 0-62mph in 7sec looks good on paper, the car doesn’t feel particularly swift on the road. The V8 is a better bet in that respect, and its guttural “woof” under power really suits the car’s bad-guy demeanour.

There won’t be a manual gearbox available with the CLS, which is fine because the seven-speed automatic transmission does a great job. It’s very eager and has a sport mode, which gives the throttle an even more aggressive response. As with most upmarket transmissions these days, you can shift gears sequentially with F1-style steering wheel buttons.

Mercedes is also sensitive to the notion that an overtly sporting appearance needs to follow through with an overtly sporting drive. To help it achieve that, you can specify the optional air suspension system, which adapts to the way the car is being driven. If you start cornering through tight bends at higher speeds, the suspension automatically firms up to stop undue roll or pitch of the body. Conversely, if you’re cruising in a straight line, the system will deliver a smooth, compliant ride.

The speed-sensitive steering does a good job of feeding back information about the amount of grip on offer, and its precision also allows accurate placing of the car through corners. Pushed hard over challenging roads the CLS exhibits a remarkable degree of composure for one so heavy. It’s unflappable, and that really is the dynamic target Mercedes needed to hit. It may lack the intimacy and, through bends, the immediacy of a BMW 5-series, but the CLS can definitely walk the walk.

So is it really a credible alternative to coupés such as the BMW 6-series and the Jaguar XK8? Well, if you believe a coupé should have traffic-stopping style and a sporting turn of speed, it would seem churlish to exclude this one from your shopping list. And those two rear doors are a significant bonus.

Just be prepared to be on the receiving end of some grief if you dare to call it a coupé.

VITAL STATISTICS

Model: Mercedes-Benz CLS 350
Engine type: V6, 3498cc
Power/Torque: 272bhp @ 6000rpm / 258 lb ft @ 2400-5000rpm
Transmission: Seven-speed automatic
Fuel/CO2: 27.9mpg (combined) / 241g/km
Performance 0-62mph: 7.0sec / top speed 155mph
Price: £41,000 (estimated)
Verdict: With sensational coupé styling and the useful addition of two rear doors, there's nothing else quite like it

THE OPPOSITION

Model: BMW 645Ci, £50,450
For: Refined engine, huge performance and great driving experience
Against: Looks quite frumpy from some angles, if not downright odd

Model: Jaguar S-type 4.2 Sport, £38,270
For: Has evolved into a much better car, with updated interior and fine dynamics
Against: Overtly retro styling beginning to look quite dated