For those who feel that the 525bhp CL 63 AMG V8 is a bit limp wristed, Mercedes has the answer. With 604bhp from its twin-turbocharged 6-litre V12, the CL 65 AMG does excess like few others. It's incredibly relaxing over long distances but is destined to remain a curious niche item.

I have a friend called Stuart. He's done rather well for himself in the IT industry and it's always fun to go shopping with him. When he's after, say, a fridge/freezer, he'll walk into the shop, ask what's the most expensive item they have and order it. He's not interested in the best value or the item that has garnered the best reviews. It's a low involvement purchase that will see him nerding out with an obese instruction manual into the wee small hours. The same buyer behaviour would seem to be the raison d'etre for the car we examine here, Mercedes-Benz's mighty, and some might add mighty profligate, CL 65 AMG.

This is not a car that, on paper, seems to make any sense whatsoever. Still, if you're intent on having the top of the range model with scant regard to the merits of 'lesser' variants, it's unlikely you'll emerge from the CL 65 AMG disappointed.

The reason why the CL 65 AMG is such an idiosyncratic choice is that there already exists a 525bhp CL model, the CL 63 AMG, that has a 6.3-litre engine and a seven-speed automatic gearbox and is really rather magnificent. Independent tests have shown that the V12-engined AMG cars are slower around a test circuit than their V8 counterparts, they sound far less dramatic and because of the massive 1,000Nm torque figure, they need a more agricultural five-speed auto to deploy the power. So what's the attraction? In short, it's the effortless straight line speed, the undemanding engine response and the sheer refinement. Whereas you'd need to rev a CL 63 AMG to a nosebleed 6,800rpm to tap into its peak power, the CL 65 AMG demands just 4,800rpm to be making its full quota.

In short, it's a lazy man's way of going incredibly quickly. The sprint to 60mph flashes by in 4.1 seconds but it's not a whole lot quicker or easier to drive than the CL 600 (0-60 in 4.3s) which costs over £40,000 less. A second generation Active Body Control (ABC) system offers flatter cornering and the steering serves up more feel than you would anticipate. The AMG spring struts provide firmer dynamics when the gearbox is switched into Sport/Manual modes. It's amazingly lithe for such a hefty car.

"Taken in isolation, it's a brilliant technical achievement.."

The aggressive front spoiler, ribbed rear diffuser, tarmac-skimming stance and huge AMG alloys instantly mark the CL 65 AMG down as something quite different to the usual CL norm. This model also gets innovative twin floating calliper brakes that are hugely expensive but devastatingly effective. The interior is beautifully styled with a move away from the confusing spray of buttons that has plagued some Mercedes models. I remember sitting in an SL 65 AMG and counting over 70 buttons and switches on the dashboard. The CL adopts a wholly cleaner look with many of the functions marshalled by the COMAND control system.

The latest generation CL is more elegant in its proportions than its predecessor, a car that could look awkward from some angles thanks to its sharply tapered C-pillars. While the latest car is less extreme in its styling, it's undeniably handsome. It's also big, expanding in every dimension to add not only gravitas, but also some welcome room in the rear seats and the boot as well. At 5,065mm from stem to stern, it's 75mm longer than the old car as well as 14mm wider and 20mm higher. This is a serious piece of automotive real estate, tipping the scales at 2,240kg. That's 530kg heavier than a BMW M6, so you can excuse it for feeling a little less puppyish.

The CL 65 AMG's value proposition is a tough one to pin down. It costs £149,565 which is serious money, even when compared to the £103,450 CL 63 AMG or the £107,095 CL 600. By contrast, the CL 500 offers as much power as most normal people would want from a luxury GT coupe (388bhp), looks a steal at just £79,652 and by most normal frames of reference is by far the best buy in the CL line up. The problem is the select group of CL 65 AMG buyers do not possess the sort of frame of reference most of us are used to. They want the top of the range Mercedes CL coupe and they buy it.

Rivals like the Bentley Continental GT and the Aston Martin DB9 decimated sales of the old Mercedes CL and the CL 65 AMG operates in a different realm to these cars. It plays by a different set of rules and faces rivals like the Ferrari 612 Scaglietti and Aston Martin DBS instead. The latest model counters with more attitude and some incredibly smart technology. I could write 1,000 words on how smart the headlights are. Then there's the brilliant adaptive cruise control that can handle the worst the M25 can throw at it without a murmur of complaint, the Advanced Parking Guidance system and the Night View Assist, an infra-red system that transmits images to the instrument panel so the car can literally see in the dark.

Although it seems vaguely irrelevant to witter on about ownership costs to somebody who is prepared to blow £70,000 more on a car that's a bit quicker and a bit quieter than the otherwise brilliant CL 500, I'll humour you and put some meat on the bones. Insurance is, predictably, a top of the shop Group 20. Fuel economy is quoted at 19.1mpg although if you can get more than 12mpg out of the CL 65 AMG on a day-in, day-out basis, you're doing better than me.

The big sting with Mercedes CL ownership comes in terms of depreciation. What seems the latest must-have can quickly be overtaken by automotive fashion. As a case study, let's have a look at a 1999 example of the then range-topping Mercedes CL 600. Leafing through a used price guide reveals the horror that the trade will offer you £7,500 for that car. Hardly a museum piece but it's worth remembering that this model retailed for £107,665 new. The CL depreciates savagely and it's best to get comfortable with that fact before making a buying decision.

It's not easy to find more enigmatic vehicles than this Mercedes CL 65 AMG. By most normal measures, it's a massively overpriced curio. Ninety per cent of what it does is accomplished by the CL 500 model which costs half as much. What store you place on that remaining ten per cent will determine whether you're one of the few CL 65 AMG buyers who are genuinely attracted by the towering straightline speed or the massive torque. It's a sad fact that most of these cars will be bought by the pornographically wealthy who possess an inability to buy anything but the range-topping version.

Taken in isolation, it's a brilliant technical achievement. It corners like no 2,240kg car has any right to, and lighting the burners on a deserted autobahn is otherworldly. Most of us, however, don't buy cars in isolation. We compare and contrast and the CL 65 AMG remains a car that's curiously unflattered by comparison.