ONE OF the first things you become aware of when you drive the 650i Convertible over a broken surface is that you have not been aware that this is an open-top car.

Typically, even most late-model convertibles display some degree of scuttle shake. You feel the body twisting, especially at low speeds when negotiating driveways. Sometimes you can feel it through the steering wheel. On the worst offenders, the rear view-mirror gives the game away.

Perhaps BMW’s latest fabric roof adds a degree of stiffness, we wondered? With the roof folded away electrically in 19 seconds, that first impression of this being an uncannily stiff convertible was confirmed.

It represents a big improvement over its predecessor and is better than the latest Mercedes E-class cabriolets in this respect. Some more seasoned journalists are generally of the view that cars need a metal roof for structural integrity but the new Six challenges this orthodoxy.

Some recent BMWs have fallen short in two key areas of dynamics, steering feel and ride comfort, but not the new 6 Series. Its fuel-saving electric power steering imparts as much feel as you could wish for in anything this side of a dedicated race car. Integral Active Steering is an option that, inter alia, also permits some rear wheel steering (below 60km/h in the opposite direction to the front wheels, 80-plus in the same direction).

And whatever happened to the harsh ride that accompanied BMW’s pioneering use of run-flat tyres? This question arose when the car was being driven (hard) in maximum attack mode aka ‘Sport +’ (the other settings in descending degrees of aggression being Sport, Normal and Comfort).

Sport+ disables the stability control, has the transmission hold onto lower gears for much longer and, delightfully, opens a flap in each of the rear mufflers (once a certain percentage of throttle travel has been reached) to allow a glorious V8 burble to emerge. Further tailoring is available via iDrive.

The point about the ride quality is that even in Sport+ it never becomes ragged or harsh. In Comfort mode, the 650i Convertible feels plush. All the drawbacks of run-flat tyres seem to have been overcome except for the need for a real spare tyre when travelling right out in the sticks.

BMW has gone to great pains to make these new convertibles more focused than the outgoing model, whose Australian debut was in May 2004. The new body is a massive 29 per cent stiffer and here is one BMW that really does deliver an ultimate driving experience.

BMW claims perfect 50/50 front/rear weight distribution and certainly there is no tendency of the kind commonly experienced in large V8-powered cars to ‘push’ into tight corners. Adaptive Drive is standard on the 650i and it eliminates body roll. BMW designers say they took inspiration from speedboat design to shape elements of the new 6 but there is certainly no float or wallow about its behaviour.

BMW’s twin-turbocharged 4.4-litre V8 delivers ferocious urge even in this 1940kg vehicle. Not many years ago a zero to 100 km/h time of five seconds was supercar material.

The eight-speed automatic transmission helps. Select ratios manually via either the magnificently tactile console lever or the wheel-mounted paddles-hifters and you’ll experience a jackrabbit effect in first and second.

There is none of that initial hesitation that blights most DSG transmissions and this unit makes a great case for the old torque converter auto. The spread of ratios is brilliant. The highest gear delivers 63km/h per 1000rpm.

Customers with upwards of a quarter of a million to spend on a car expect to be able to choose from a wide range of options to add extra individuality and luxury and BMW does not disappoint with bespoke features for the new Six, albeit at quite a price.

Soft Nappa leather for the dashboard, centre console and the tops of the doors costs $3000. Add contrasting stitching for a further $500 – nice, but you’d want to be sure of hanging onto the car for quite a few years to justify such extravagance.

The 640i is equipped with lightweight sports seats, with are a no-cost option in the top-shelf 650i. Acknowledging that not all customers will want them, the Lane Change Warning System and Lane Departure Warning System are also (extra-cost) options.

Our test car was trimmed with beautiful polished American Oak with lots of highlights. It also had the lavish Nappa options, but the standard dash does not look in any way cheap or in need of extra adornment. The real wood did not add to the cost of the vehicle, ‘fine brushed aluminium’ being the standard finish.

The interior is beautifully executed. The large screen is well integrated and the whole appearance is stylish, functional and very classy. The ergonomics are mostly logical - you can swap between Sport+ and lesser settings without taking your eyes off the road - but iDrive still presents various issues.

The head-up display is a welcome standard inclusion, the 650i also gets a rear-view camera (cleverly hidden behind the swiveling BMW badge in the bootlid), which is especially useful when the hood is down, and BMW’s clever new Surround View should reduce the risk of scratching those expensive 19-inch alloy wheels.

Brake energy regeneration, electric power steering, the de-coupling of the air-conditioning and oil pump from the engine when they are not in use are examples of the manufacturer’s EfficientDynamics technologies.

Two golf bags can be fitted into the boot with the hood up. Maximum volume is 350 litres, with 300 available when the soft-top is stowed.

Overall, the BMW 650i Convertible is a compelling sportscar of notably high quality. It accommodates four adults and quite a bit of luggage while all but matching a Porsche 911 for verve.

The biggest challenger for the 650i is almost certainly going to be its six-cylinder sib, the 640i, which is set to arrive at $194,300.

BMW Australia hopes that, while the V8 variant shakes up the 911/SL/XK Jaguar market, the 640i will tempt would-be buyers of an E500 Cabriolet to find an extra $5K or so for the BMW, which is really in the next class up in terms of size and presence.

Rapid and fuel-efficient as the V8 is, the addition of merely seven-tenths of a second for the 0-100km/h sprint and an improvement in economy from 10.7 to 7.9L/100km has the 640i looking like the shrewder purchase in difficult times.