I always find it hard to know what to think about Mercedes. They are, in their sleek, efficient way, oddly contradictory. On the one hand, the juggernaut-like S500 is the ultimate chief executive express - a car that seems to float on a magic carpet of surplus value as it whisks captains of industry from Learjet to boardroom. And there's the SL convertible - a real "look at me" Sunset Boulevard cruiser in its luxury version.

Then, on the other hand, there's the fact that in every city from Damascus to Delhi the stalwart of the taxi fleet will be a dusty yet ageless Merc saloon. And the fact that, on any given weekend in Bucks or Berks, you cannot move for Mercedes estates. Couth but not flashy, these too act as a kind of taxi fleet, while their owners ferry offspring to and from the social hubs of the English home counties. (The labrador in the back comes as standard.) And, finally, you find the Mercedes much parked on gravel drives of the not quite in the country but definitely far from the madding crowd houses of the affluent retired.

So what does all this add up to? Well, it's posh, but it's also a workhorse. It's German, and therefore engineered to the nth degree. It's definitely more upscale than a VW, yet it has neither the aggression and attitude of a BMW, nor the techno-modernity and metropolitan chic of an Audi. The Mercedes advertises itself as classy, but only diffidently so, with an air of well-bred insouciance.

Sure, it speaks of money, yet at a subliminal level: which colour would you like yours in - silver or gold? These are the colours of currency. Or black, as in "black gold". But unlike other luxury items, the Mercedes does not seem a frivolity or an extravagance. Somehow, it presents itself as a gilt-edged bond that will last and keep its value. Rather like a Volvo, but with a subtle touch of glamour.

Meanwhile, the Merc's familiar face - the paired headlights on either side of the radiator grille - is one of the most unchanging and characteristic in automotive design. It is as if it is saying: "Be reassured - Mercedes-Benz believes in traditional values, traditional virtues. Let Mercedes-Benz be your still point in a turning world."

And reassuring it is: the door closes behind you with the kind of solid, emphatic thunk you'd expect to find in a bank vault. Inside, you are enclosed in something between an airlock and a time warp: a coach-built interior of wood, chrome and leather that harks back to the last days of the Austro-Hungarian empire. The knobs and dials are discreet and sensible, with no concession to any "designer" preference for style over substance.

The gearshift is, of course, automatic. Mercedes-Benz does "do manual", but only in the way that Sir Simon Rattle would "do" a Beatles tune - for an encore, perhaps, to amuse his audience. The only slight dissonance here is that, if you have the window down, you eventually realise that the diesel-engine chunter is actually coming not from the van sitting next to you in traffic but from your own Mercedes. I never quite understand why anyone bothered by the price differential between petrol and diesel would fork out the £28k asked for this model, plus whatever the insurance premiums would be. But there are lots of things I don't understand, and apparently it happens.

In any case, with the windows up, you can pretty much forget about whatever DaimlerChrysler put under the bonnet. It could all be done with elastic bands for all you care. You just put the thing in drive and off you go, in a pleasantly somnolent, barge-matic sort of way. It actually feels like a bigger car than it is, but with very light steering, a surprisingly tight turning circle, and extremely sensitive, powerful brakes - making driving it about as demanding as taking a bath.

Basically, there are two modes of progress in this Mercedes: pootling and wafting. It's so far from being a driver's car that it has come out the other side. The thing is, despite myself, I was more and more persuaded by this. Like, what is the rush? Yes, all right, it's a bit of an old dad's car... so call me an old dad, but there are worse things than arriving at your destination with your blood pressure bang on normal.

And then there's all the clever stuff the car does for you. The solar panel in the roof that helps keep the interior air-conditioned even while the car is parked. Cruise control and a navigational system that has a dual display (one in the centre of the speedo), so that you're not constantly looking down at the central console instead. The usual electronics keep the car upright and on the road, and an automatic system triggers the seatbelt tensioners and side airbags if they don't. And when it rains, the brakes will gently apply to dry the discs without the driver even knowing it.

Which is typical. Driving this Merc is the closest you can come to being chauffeured without actually paying someone. And when you think of how much you'll save on a chauffeur's salary, the price starts to seem a bargain. The seduction is complete.