I always look forward to driving cars from Mercedes-Benz’s super-high-performance AMG division, but perhaps not for the most obvious reasons. Clearly there’s the speed thing but, for me, that’s never been enough. If it were I’d live in France and travel everywhere by TGV.

Mercedes-AMG is part of DaimlerChrysler and is located in Affalterbach, Germany. It specialises in the development and manufacturing of high-performance Mercedes-Benz vehicles. But the truth about AMGs is that you never quite know what you’re going to get. Like food in a Michelin-starred restaurant, the menu will tell you what it should be like, but the only way to be sure is to taste it.

My most recent exposure to AMG products — and if you count the AMG-powered SLR McLaren, there are 15 different versions to choose from — supports my view that the more powerful and expensive the AMG, the less tasty it will prove.

You can, for instance, spend £145,240 on a 6 litre turbocharged V12 coupé with 612bhp called the CL 65 and discover you’ve just bought the world’s fastest sitting-room. Or you could leave nearly a hundred grand in the bank, splash out £49,640 on this SLK 55 AMG, and drive one of the sweetest, most appealing roadsters to cross my path in many years.

This SLK doesn’t need 12 cylinders or a turbocharger to work its magic. Its 360bhp 5.4 litre V8 engine is the least powerful AMG engine on offer by a margin of well over 100bhp. The result should feel like AMG lite, an unsatisfying pale imitation of the real thing. In fact it’s the best AMG product I’ve tried.

In part this is because the host vehicle is so capable. The AMG is built around the SLK 200, which is still the best all-round roadster that that amount of money will buy. Unlike its predecessor, the worthy but somewhat dull original SLK, the second generation car is as good to drive as it is to look at. The folding steel roof is still the best in the business and those of us who are 6ft tall or more can at last get comfortable inside.

The AMG bits transform the SLK from marvel to near miracle. The engine is actually a close relative of that used in the £313,645 SLR McLaren and while it lacks superchargers and an outrageous output, the quality of its power delivery is as good as any.

It sounds fabulous: a deep, mellifluous V8 thunder, and by any standards other than AMG’s it delivers knockout performance. This means 0-62mph in under 5sec and a top speed that would easily top 170mph were it not for electronic restriction at 155mph.

No Porsche Boxster would see which way it went. More impressive is the fact that the SLK 55 is both more powerful and quicker off the mark than a new Porsche 911 Carrera Cabriolet, which will cost around £15,000 more than the SLK when it goes on sale later this year. Sure, the Porsche has some tiny rear seats, but the Merc has a folding steel roof and terrific looks.

In fact it’s extraordinarily difficult to find a true rival for the SLK 55.

A TVR Tuscan S is similarly priced and a little more powerful but it’s hard to imagine anyone tossing up between such a brutal British bruiser and a svelte sophisticate like this. The Alpina (a specialist BMW tuning company) version of the Z4 is a fine car, but in power and price it is in the class below the SLK 55.

While this grants the Merc a certain exclusivity, that’s not the root of its appeal. What makes this car so damned attractive is that it works as well in the real world as it does on the specification sheet. Its speed is a side issue: of more importance is the fact that it handles beautifully and rides well enough to be a natural everyday car.

It will, of course, put the wind through your hair if you want, but when that roof snaps shut the car appears to have the structural integrity of a bomb-proof safe. It will appeal both to technophiles — its automatic gearbox has seven gears — and sybarites who want nothing more than to sit swaddled in leather, sailing on an ocean of effortless torque while they fight off admiring gazes from the kerbside. And yes, it goes like stink.

Taken as a whole, it gels better than any other remotely similar roadster. A Boxster has more finely resolved handling, a Tuscan greater outright acceleration, but as a thing to live with, in all seasons and on every day of the year, it’s out there on its own. I guess I’d rather have its big brother, the astonishing SL55 AMG, with its storming, supercharged 500bhp motor, but it’s almost exactly twice the price and, good though it is, it ain’t that good.

That is why the SLK 55 joins the Mazda MX-5 and Lotus Elise on a very short list of cars that can credibly claim to be the world’s best open two seater. This summer I expect them to be joined by the Ferrari 575 Superamerica, but if your budget is anywhere near £50,000, right now you won’t do better than this.

Vital statistics

Model Mercedes-Benz SLK 55 AMG
Engine type V8, 5439cc
Power/Torque 360bhp @ 5750rpm / 376 lb ft @ 4000rpm
Transmission Seven-speed automatic
Fuel/CO2 24.1mpg (combined) / 288g/km
Performance 0-62mph: 4.9sec / Top speed: 155mph
Price £49,640
Verdict A strong candidate for world's best roadster
Rating Five stars

The opposition

Model Porsche Boxster S, £38,720
For Sublime handling, improved looks, residual value
Against Not that attractive, cabin cramped for tall drivers

Model BMW Alpina Roadster S, £38,000
For Better in almost all ways than the Z4 it's based on, and rare
Against Likely to depreciate faster than mainstream rivals