Mercedes has shifted an incredible 308,000 SLKs in the seven-and-a-half years since its rapturous 1996 launch, and it predicts its replacement can do even better. Way too ambitious, I thought, when I first saw the new SLK at a sneak preview in December. However, for me, the shift from sceptic to believer required no more than one terrific mountain-road drive. You see, the new car’s abilities go way beyond the SLK’s Great All-Rounder tag. From secure coupé to open roadster in just 22 effortless seconds, the desirability of the roof concept is truly inspired and remains one of the SLK‘s unbeatable virtues. About here, I can hear you muttering dismissively, ‘so what, doesn’t that make it a hairdresser’s car?’ It’s true the SLK still occupies a unique niche and that no rival attempts to cover as many bases. Thing is, the new car succeeds brilliantly, without any fundamental change in its character or purpose. It’s grown up, more mature, faster, better to drive, roomier, has even better build quality and, probably, the best open-car structural rigidity. What it doesn’t do is set out to be as sharply focused as, say, a Boxster. Yet think about it: at launch the first SLK’s top engine was a gruff 190bhp supercharged four-cylinder driving an automatic gearbox. The new car stretches to a 355bhp 5.4-litre V8. Today’s entry-level car is a supercharged 161bhp 1.8-litre four, and the mid-range engine is the new 3.5-litre quad cam V6 – the car I drive here. And you have the choice of six-speed manual or seven-speed automatic transmissions. It’s pelting down in Majorca, the roads are desperately slippery and our SLK 350 treats the conditions with disdain. In terms of refinement, ride quality, engine flexibility and comfort, we might as well be driving an E-class. Mostly, however, I’m taken by the breadth of change to the interior. This is the best-finished Mercedes cabin in a decade. Best news is that the extra 30mm of wheelbase has greatly enlarged the cabin. Tall-driver comfort is no longer compromised. My 6ft 4in frame demands moving the seat forward a notch in order to fully depress the clutch. Add a steering wheel that’s now four-way adjustable and multi-adjustment for the seats, and it’s easy to find exactly the right driving position. My senses are being attacked from every direction. I can’t quite believe the ride quality. The C-class-derived suspension is soaking up the bumps, completely filtering out small irregularities. It feels too soft, too supple for a sports car. I begin to suspect the SLK could come over as a boulevard cruiser. The new engine is gutsy down low, with vastly better flexibility than the Z4 3.0. My delight at finding a new V6 that’s silky smooth and powerful, with a sonorous exhaust note, is tempered by the low-for-a-sports-car 6200rpm red line. Just as the engine is hitting the high notes, pulling ever faster, it runs into the gentle 6400rpm cut-out. And forces a gearchange. At last, a Mercedes manual gearbox that encourages the driver to shift gears. First to second and return is still a bit baulky, but between the other ratios the little lever’s swift, mechanical movement is a vast improvement. Not as short or light of travel as an MX-5’s, but pleasurable. Point is, it’s now good enough to offer an alternative to the terrific new seven-speed auto. The SLK is the last current-model Mercedes to adopt rack-and-pinion steering and the advantages over the old recirculating-ball system are immediately obvious. The fuzzy on-centre feel has disappeared. The steering’s not super-sharp or extra quick around the straight-ahead, but the weighting is just right: there’s no friction and it’s utterly consistent and linear in its reactions. You quickly take the new agility for granted. Even on Majorca’s notoriously slippery roads, grip is impressive, the ESP warning light only rarely cutting in, and then without abruptly spoiling the fun. And it is fun – far more satisfying to drive than the old car. Day two. The sun is shining, the mountain roads are tight, and the SLK is flowing from corner to corner, never pushing the nose wide, but also, and even when provoked, refusing all attempts to kick the tail out into power oversteer. It won’t budge, even at full noise in second-gear corners. Steering turn-in that is honestly fluent rather than instantly eager perfectly suits the SLK. What really works is the ride produced by the front strut/rear multi-link suspension. The chassis ignores mid-corner bumps and settles the body quickly, so although roll is evident from outside the car, the driver remains unconcerned. If your definition of a sports car includes a taut ride you’ll likely dismiss the SLK. The rest of us will relish its ability to disregard nasty surfaces – which surely helps in eliminating scuttle shake, as the thing always feel rock solid – and just get on with the task of travelling quickly. Which, as a result, comes naturally and without upsetting your passenger. All this means exploiting the V6’s 268bhp and 258lb ft of torque – from 2400rpm to 5000rpm – couldn’t be easier. Play lazy, short-shift and draw on the mid-range and the SLK is spirited, utterly effortless. Press harder and it lunges at the horizon. This, the mid-spec version, cuts to 62mph in only 5.6sec, undercutting the old SLK 320 by 1.3 seconds, and is now quick enough to seriously worry a Boxster S. The SLK 55 AMG, due this summer, takes just 4.9sec. The new, bigger, better-equipped, 1390kg SLK’s put on 60kg over the old V6, but this obviously has no impact on performance. Top speed climbs by 3mph to a limited 155mph. Constant gear-swapping reveals the ratios are more widely spaced than expected on a sports car. Yet, for most drivers, this is unimportant, given the level of torque and power. More critical is the car’s cruising refinement, low noise levels and a real ability to cover distances rapidly, a virtue that’s ably assisted by the potential range offered by the now 70-litre fuel tank, a whole 17 litres up on the first car’s. Apart from big brother SL, no other sports car asks so little of its owner. In purpose it’s subtlety different to the many rivals, but that doesn’t mean it’s any the less effective as an enjoyable driving tool. The number of competitors may have doubled in recent years, but I suspect that if enough people like the way it looks (they’ll surely love everything else about it) the SLK is not going to have any trouble achieving Mercedes’ sales ambitions.