WE ARE not the only ones to be taken aback by the arrival of the Mercedes-Benz R-class. It's big, seats six and has all-wheel drive, but isn't designed to go off-road. So what's the point? Is it just a cynical marketing exercise designed specifically for the US market, where they love big things, or has Mercedes-Benz stumbled on a niche that had been overlooked? After spending time in the R-class range and more recently the superb turbo-diesel R320 CDI, it's clear this vehicle makes much more sense than a large four-wheel drive. Joining the R range on sale this month, the CDI is the pick of the R-class range that went on sale here in April. It is one very sensible family wagon.