So when I heard that another M3 was arriving I reckoned that the chances of getting a quick blast were remote. That was until I saw it pull up. First of all it was a convertible - never a winner in the office. The colour didn't help. Laguna Seca Blue isn't too offensive on the subtly aggressive coupe but on the convertible, in combo with optional glitzy alloys (huge 19 inch diameter), the convertible looked every-inch the pimp-mobile. Peter declared that he wouldn't be seen dead in it. John was on holiday, Dickie grimaced at the thought of the biggest alloys on the wobbliest model and Harry didn't mutter a word. I donned my best gold chains and prepared to be subjected to some unwanted attention.

Happily you can't see the questionable aesthetics once ensconced in the rather more tasteful, black leather trimmed interior. From the driver's seat it's business as usual, which means perfect ergonomics, superb quality and a general feeling of well-being, and that continues when you fire-up BMW's latest straight-six masterpiece. We've talked a lot about this superb engine but it's great to jump back into an M3 and revel in the smoothness, power and noise that the 3.2-litre six generates. It may be 'only a 3-series' to some, but the M3 comes with one of the truly great supercar engines.

Hood down on a smooth, sweeping A-road, you could almost be forgiven for choosing the convertible over the coupe. The chassis feels admirably shimmy-free, the engine sounds even more spectacular than I remember and acceleration isn't perceptibly dulled. According to BMW's figures, it should be good for 0-60 in around 5.3secs (we recorded 5.1secs for a the coupe) while top speed is limited to 155mph.

However, put it on your favourite stretch of B-road - the twisty, bumpy sort littered with off-camber corners and tightening bends - and gaping holes begin to appear. Wicked mid-corner bumps that the coupe would shrug off elicit shudders and groans of protest. Sharp ridges send an almighty jolt through the body and screen surround, encouraging a slightly gentler pace. BMW is not a company to cut corners, but despite a heavily reinforced windscreen frame and strengthened floorplan, the convertible cannot hope to match the structural integrity of the coupe. The steering highlights this, kicking back severely when the going gets really tough.

Those 19in wheels don't help. The ride is by no means destroyed but certainly suffers. Both urban and fast open roads reveal a noticeably more fussy ride which soon becomes a bore. The convertible is less of a weapon and more of a cruiser.

Don't think that the M3 has gone completely soft. Most of the time it remains a very fast and effective tool. But it's that last 15 or 20 per cent of ability that defines the Motorsport BMW. That the convertible becomes less convincing on the sort of road where the coupe excels, and from which we all derive so much fun, is a real shame. Predictable, but a shame none the less.

If you're rubbing your hands together in glee, waiting for the first time you come across an M3 convertible driven by someone more interested in posing than power-sliding, hoping to humble him in a more affordable pocket-rocket, don't get too carried away. Even in inexperienced hands the M3 is blisteringly quick, soft-top or not. The traction control system is well-judged - allowing just a little tail-slip before cutting in rather than spoiling the fun before it has even begun. Turning it off results in as much tail-out tomfoolery as anyone could possibly want, but either way you'll be making rapid and fluent progress. And enjoying yourself.

The gear change can feel a little vague and rubbery at town speeds but smooths out when used briskly. Steering feel is disconcertingly light initially but still manages to convey the vital messages about the road surface and the onset of understeer or oversteer. Normally it's the latter, metered out in progressive, satisfying style by the smarty-pants M-diff. Heroic car control skills aren't required to make the M3 dance. Forgiving yet involving, the M3 chassis is about as good as it gets.

But here's the rub. When choosing an M3, surely chassis dynamics are right up there with performance on the list of reasons to buy. If I was helping fund all that development work on the 'Ring, I'd like to think that I had the ultimate expression of BMW's know-how. With the convertible that isn't the case. Each evolution comes with claims of greater structural rigidity - compared with the previous convertible the new car is said to be a huge improvement, with torsional rigidity up by 20 per cent. Trouble is, the coupe is making strides just as fast, and highlights the inherent drawbacks of chopping the roof off.

Despite a snug hood which manages to keep wind noise to a minimum and does a good impersonation of a hard-top most of the time, when that dream road snakes out into the distance, distractions like scuttle shake and compromised steering become a nagging reminder that the coupe is still the M3 to have.