I've personally driven an S Class (the S320 CDI to be precise) and I must say I wasn't impressed with it as a driver's car. Yes, it is a magnificent cruiser and the most comfortable car I've ever been in; but you just don't get much feedback from the car, it's just too "neutral" when cornering, and that in my opinion doesn't make it a very safe car to drive through a twisty road at high speeds. It does behave better in a motorway, and 180-200km/h cruising is within easy reach.
But anyway, an Audi A4 TDi is a more satisfying car to drive in my opinion. It lacks a bit on power, and the brakes are not top notch in terms of stopping ability, but it has very high cornering limits and conveys much more information than the Mercedes.