How do you like your M cars? The original and now highly sought-after four-pot M3 nailed the template\: raw but thrillingly interactive. Various M5s over the years moved the M dial further into seductive stealth mode. Of the current breed, 2010's M3 GTS staked a big claim for being the best ever, with a nod to the equally old-school 1M Coupe.The suave new M6 convertible is an entirely different proposition, and a bit of a head-scratcher. Not on paper, admittedly, where the numbers are irresistible. BMW says it's more powerful, more athletic, more focused, more luxurious and more efficient' than before, some of which sounds a little contradictory to me. As is the modern way, it does indeed deliver more of everything while using less natural resources and emitting less (it'll average a claimed 11.6kpl and cough out 239g/km of CO2). Good to know, though probably not an issue for prospective owners for whom global capitalism's tsunami is a mere pond ripple.It looks great, roof up or down, especially in BMW's currently expressive colour range. The juicy bit, though, is the engine. This is the same 4.4-litre twin-turbo V8 that's already blown us away in the latest M5. Peak power is 552bhp from 5,750-7,000rpm, but better even than that are the 680 torques it summons up from the bowels of hell from just 1,500rpm, an amount it then generates consistently through to 5,750rpm. So there is grunt aplenty, deployed in a way that makes it roughly three times as flexible as the surprisingly peaky and oddly irritating old V10 M5 engine.It's also cleverly packaged, with an eight-into-four exhaust manifold, twin-scroll turbos (sharper throttle response, less lag), direct injection, Valvetronic and double-Vanos valve-timing. Though there's 10 per cent more power and 30 per cent more torque than previously, fuel consumption is reduced by 23 per cent. The 0-100kph sprint takes 4.3 seconds, and the limited 250kph top speed comes around not long after that.That's the easy bit, believe it or not. To get a true handle on the M6 beyond this would be a challenge for Bill Gates himself. That's mainly down to the world of possibility that modern chassis electronics opens up, a world BMW's engineers have elected to travel with the keenness of latter-day Marco Polos. First up is a new electronic, multi-plate diff, whose control unit is in constant dialogue with the DSC stability control, and also cross-checks data drawn from the throttle pedal's position, the wheels' rotational speed, and yaw rate. Zero to 100 per cent lock-up is determined within a fraction of a second.There's more. The (hydraulic) power steering, suspension, transmission, throttle response and traction control can all be configured according to taste, in almost 500 different combinations. The driver can then store a get-me-home-in-the-rain' setting and a get-me-to-the-nearest-empty-roundabout-for-tyre-shagging' setting on the two M buttons on the steering wheel. There's Comfort, Sport, Sport Plus and M Dynamic Mode (MDM), all of which adds up to a car with more chips than Scarborough seafront.But, as anyone familiar with music-streaming service Spotify will attest, there is such a thing as too much choice. This silicon-driven overload manifests itself on the road, where the true identity of the M6 convertible struggles to assert itself. Fundamentally, it's sorted. Its chassis is unbelievably rigid, and it rides beautifully, two things that even in a 2012-era BMW soft-top you shouldn't take for granted. It's also blisteringly fast, the brakes are superb and its seven-speed dual-shift transmission is velvety smooth in full auto, boulevard mode, but equally effective if you opt for maximum attack.There's no lack of grip even with all that power and torque trying to peel tarmac, and if you overcook it in a tightening corner, the electronics intervene with such alacrity that only the tiniest muscle in your backside will have an inkling that the machine part of the man-machine interface has just saved your bacon. Sounds extremely impressive, doesn't it?The problem is the familiar one, the one that no end of Mensa-level boffinry can solve\: character. Objectively speaking, rivals like the Maserati GranCabrio and Aston Vantage roadster don't have anything like the ability of the M6 convertible, but both administer a compelling low-level erotic charge the analytical BMW simply doesn't. The latest Porsche 911 manages to be efficient and soul-stirring, while the Jaguar XKR-S is crazy fast and twice as exciting.Finally, there's the small matter of the M6's cost. This is a thumpingly expensive car. It's also undeniably a master-class in modern automotive engineering. But that, for a whole load of ridiculous reasons, doesn't automatically mean it's also the best.Jason BarlowThe numbers4398cc, V8, RWD, 552bhp, 680Nm, 11.6kpl, 239g/km CO2, 0-100kph in 4.3secs, 250kph, 1930kgThe verdictObjectively and technically speaking, BMW's new M6 Cabrio is a riotous barnstormer. But the fizz is just a little bit flat.