Usually the first thing I do when I get into a new car is to press all the buttons to find out what they do. (This got me into trouble once with one marked "SOS" in a new Peugeot, but that's another story.) But, with a BMW M5, there are all sorts of other childish pleasures to be had - such as driving the thing as fast as your bladder will allow - so at first I didn't take much notice of the small, ordinary-looking, steering wheel-mounted button marked, simply, "M".

Consequently, for some time I firmly believed that the M5 was quite the worst £62,000 car I had ever driven, and worse than a good many cars less than half that. The paddle-shift gear change was jerky, like the motion of carriages when a train starts to move; the steering felt over-light and disconnected; there was pronounced throttle retardation even when changing up; and, most bewildering of all, the M5 didn't seem all that fast. I was not impressed.

Actually, I had disliked it before I had even climbed in. The 5 series is the least successful of Chris Bangle's designs for BMW. It is graceless, heavy, jowly and aggressive, exuding the worst kind of automotive karma. This is a car for people (let's be honest, men) who bully office underlings and have crushing handshakes - the "M" even more so. The self-conscious gigantism extends to the panel gaps which, on my M5 at least, were unusually wide for a quality German saloon. BMW is now one of the most profitable car companies in the world, I thought to myself, so surely they must be doing something right. But what?

One of those things quickly becomes apparent as you settle into the driving seat. The cabin is a masterpiece of bare-surfaced simplicity, fashioned from robust, quality materials. There are no ruched leather-door trims, over-varnished walnut dash inserts or half-timbered steering wheels to be found here. The only real let-downs are the cheap plastic gear-change paddles. They are mounted on the wheel itself, so they move as you steer and you can't rely on them being in the same place twice - the column-mounted, leather-faced, cold chrome paddles found in Bentleys and Astons are infinitely preferable.

Other than that, purpose and focus are the main watchwords for the M5, which perhaps explains why that "M" button is so discrete. Eventually, in an idle moment, I pressed it. All hell broke loose or, to put it more accurately, 507 horses as, unbeknown to me, up until that point I had been making do with just a mere 400bhp.

Not only did the button unleash true supercar levels of power - the power to give you an instant (if sadly transitory), face-lift; send a surge of fight-or-flight chemicals coursing through your body; and reach that next 90-degree corner immediately, but it also cured, in an instant, all the other gripes I had about the gear change, steering, throttle response and ride that had soured the M5 experience thus far. The M5 had come to life, as if given a jolt from Dr Frankenstein's lightning conductor, to the extent that, hilariously, when cornering, the electric seat bolsters began to squeeze my kidneys to hold me in place. It was like being hugged by a Gruffalo.

All of which leaves me feeling rather puzzled. If the M5 has the potential to be this great, why not have greatness as the default setting?

It's a classic: BMW M535i

The remit for the first true, road-going BMW M car (not counting the more rarefied M1 supercar) was simple yet audacious: to offer supercar performance in a family saloon. Branded "M" for motorsport, the original M5 defined an entire new automotive genre by taking the standard body shell of a production saloon and planting in it a far more powerful engine, close-ratio gearbox, powerful brakes and sports suspension. Back in the 1979, when the first M535i was built, it was normal for performance versions of production cars to sprout all manner of wings and spoilers to signify their ability, but the M5 wore its power discretely - the only real exterior tell-tales were a deeper front air dam and alloy wheels. Inside, little belied a top speed close to 140mph and a 0-60mph time faster than contemporary Ferraris. A few aped the M535's format (Mercedes with the 500E and later AMG versions of its cars, Audi with the RS6 and Jaguar with the XJR), but, 26 years on, the new M5 remains the benchmark.

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