BMW X5

With a 0-60mph sprint time of only 6.2 seconds, and looking meaner than an M5, with its flared wheelarches, special paint and huge 20-inch alloys, the iS certainly fits the bill both on the road and on paper.

The 4.6-litre engine is a beefed-up version of the standard car's 4.4 V8, with a sportier character. More hot-rod tricks are used in the five-speed auto box, which is tuned to shift at higher revs, and has different gearing to allow a greater top speed.

But can a big 4x4 really behave like a sports saloon? Down country lanes, the new model revs noticeably quicker than the 4.4 model, and feels substantially more eager from a standstill. Despite this, it doesn't seem as rapid as the similarly powerful Mercedes ML55 AMG, even though the Beemer has a quicker time to 60mph. Perhaps that is due to the X5's surprisingly muted exhaust note, which is a disappointment on a car of this character.

BMW has firmed up the suspension of the 4.6 iS, too. The first thing you notice is the steering, which initially comes across as remarkably like that of the 5-Series - crisp and linear - but is almost uncomfortably light for a car of this size, despite having been tuned specially for this new model. However, it helps move those big tyres, which in turn tame the upgraded brakes. With huge 14-inch discs up front, the iS's anchors are bigger than a supermini's wheels! As you'd expect, they haul the X5's substantial mass down from speed with little effort.

This accelerating and braking prowess goes to prove that a big, tall car can cover back-roads at the same speeds you'd reserve for an M5. The same goes for the cornering ability. In fast bends, this X5 understeers slightly, but lift the throttle and the back end will step out just like an M5, even with the back up of a DSC stability system.

All of this behaviour, plus the high-level price, makes us think the top-performing X5 should have been called an MX5, but the purists at BMW claim M cars are only for the track, not the trail. Besides that, Mazda's copyright lawyers are watching closely...

The iS is to the X5 range what the M3 is to the 3-Series. Besides trouncing the Mercedes ML55 AMG, its performance and handling will shame many sporting saloons. It looks great, too, but the suspension and wheels will restrict the X5's off-road capabilities. Despite the iS's excellence, we'd opt for an M5 instead.

At a glance

* Performance version of X5 4x4, on sale in UK from February

* Priced at ί¿½54,000, with extra equipment over 4.4 as standard

* V8 unit bored out to 4.6 litres