The convertible has got to grips with the handling issue of its predecessor, making it a contender for class leader.
The revised BMW 6 Series is six times the fun
It doesn't take long to work out that BMW's all-new 6 Series convertible is a massive step from its predecessor.
Really, the first corner with a bump halfway through it will tell you most of what you need to know. The old car hated this kind of corner and complained loudly and angrily about being put through it. This car, built around the chassis architecture of the new 7 Series and 5 Series saloons, is a whole different animal.
It's so much better that it's become an instant contender for class leadership, which the old car never claimed.
It doesn't hurt that it's fast, either, because the twin-turbo V8 version (which was the only one we drove) can rip to 100kph in five seconds flat on its way to a limited 250kph top speed.
Yet the old car was quick enough, but because of the combination of enormous weight and a big hole where the roof used to be, it struggled to keep its suspension working the way it was designed to work. And that meant it didn't have as much Sheer Driving Pleasure as anybody wanted.
The new 650i Convertible is still no lightweight (in fact, at 1,940kg, it's actually 20kg heavier), but BMW has done such a good job of tightening things up below decks that it claims the car has a full 50 per cent more torsional rigidity.
That explains why, when we cocked the big 6 over hard onto its suspension on a sweeping South African bend and aimed it, deliberately, at the biggest bump we could find, nothing unusual happened. The car sat there, weight pushing down hard on the outside tyres, perfectly on the line we asked it to take and sending just a vertical thump through the seats. In the old car, there'd have been a massive wobble as the bump sent a shockwave through the body and the whole thing would have stepped sideways.
No, it's a completely different machine, this, and with Mercedes-Benz's SL entering its dotage and Audi's S5 just a bit smaller, it's really only contending with Maserati's superbly emotional GranCabrio and Jaguar's visually crisp XK convertible.
Visually, it's a cleaner design than the sometimes jarring old car, too. Externally, you'll love it or you won't, but the cabin is a bit more special than most cars in the class.
The seats are very comfortable and offer a lot of lateral support, too, and it's very quiet with its three-layer roof latched on. It's a very simple system to use, with just one button, and it can be operated at up to 40kph. BMW claims it opens in 19 seconds and closes in 24, but we couldn't get within three seconds of either claim.
Inside, though, there's a wonderful curve that starts in the doors and stretches up into the dashboard, and it's all topped off with a distinct, curving line that shows BMW is back to having the driver's comfort as its main focus.
Though not quite as practical as the 5 Series cabin, it's definitely more stylish and, if anything, the materials it uses are of even higher quality.
BMW claims it's a full four seater, but beware the wrath of any adult you place back there for more than 15 minutes or so. Maserati still owns the rear seat honours.
Yet the bigger problem is going to come when you hit the motorway four-up, because BMW's ungainly, fold-down wind blocker sits right about where the rear-seat passenger's torso would need to go. Put anybody back there and everybody suffers more wind buffeting than they'd like, even though the glass rear window winds up and down to help cut the buffeting.
The shame of it is that the click-in, fold-down wind blocker is not as elegant a solution as the body shape or the interior deserves, especially not when Mercedes-Benz has the wonderfully effective Air Blocker, which pops up from the top of the windscreen, already on sale in its much-smaller E-Class Cabrio.
Still, even when the roof is folded down into its cubby hole, BMW claims that the boot will give you enough space for two sets of golf clubs and that looks like a fair claim.
There's a thick, sporty steering wheel dominating the smooth new dashboard and about the only jarring visual note in the cabin is the new, super-sized MMI (Multi Media Interface) screen that juts up from above the air vents in the centre.
It might not be pretty, but it's huge and it is easy to read from any of the four seats.
But the seat you'll want is the driver's one because that's where you get to play with the 6 Series Convertible's suddenly agile chassis.
It has a lot of toys to play with, including the ultra-adjustable Dynamic Drive Control (which gives comfort, normal, sport and sport plus modes), complete with electrically adjustable damping and anti-roll bars, standard.
It's clearly an American-aimed machine, yet it does everything pretty well because the swoopy new interior is more driver-focused and so is the rest of the car.
The chassis is well balanced and when you fling it at corners, you find it sticks with admirable commitment. Keep pushing, and you find it corners flat, doesn't suffer any frightening body roll when you quickly change direction and it just grips and grips.
For the most part, bumps don't affect it unduly, either. For the most part. Hard, sharp hits can send it into a sharp skip (all four-seat convertibles do it), but the stiffer chassis helps to rein that in quickly, without sending a shiver up the thick A-pillars.
Come in to a long corner too hot and the car has enough balance to let you adjust it, time and again, as you fiddle with the throttle, the brake and the steering.
The brakes, too, are another highlight. They are incredibly strong (they need to because they've got two tonnes to arrest), but more importantly, the pedal gives tremendous feel and a beautifully linear progression as you ask for more stopping power.
But if the chassis's new-found core competence is the surprise, the engine-gearbox combination is the star.
The twin-turbo V8 is superb, steaming to 100kph in 5.0 seconds and proving superbly flexible from most rev ranges, thanks to its 600Nm of torque and 408hp of power.
And, oh, does it sound good. When you're cruising with a constant throttle (or on cruise control), it's very quiet, leaving you with just the rush of wind around the A-pillars. When you ask it to rush, though, the engine note becomes deep, burbly and unmistakably a V8, even with two turbos muffling the music.
The eight-speed gearbox is brilliant, too, whether you're shifting manually on the paddles or leaving it to its own devices. Each shift is super smooth and even on the sportier modes, when it's supposed to shift more aggressively, it still doesn't disrupt the cabin comfort.
It's also quiet when cruising, and the engine's superbly deep note only intrudes when you ask it to. Oh, and it only uses 10.7L/100km.
Something else you should know?
While we had no chance to drive the smaller-engined 6 Series Convertible, the 640i, there's every reason to believe it might actually be the pick of the two cars.
Its TwinPower turbo six has 320hp of power and 450Nm of torque, but it's a full 100kg lighter (though 1840kg is still not inconsequential).
It's only 0.7 seconds slower to 100kph (so 5.7 seconds, which isn't bad), but uses 7.9L/100km (nearly three litres better) and emits 185 grams of CO2/km.
And, BMW's engineers insist, it handles far better because most of the weight savings are concentrated over the front wheels.