Slim,
I test-drove a diesel one of these last week (C220 CDi). 100k miles on it, and no squeaks or rattles. Automatic was quite good, you can drive in automatic or manual mode (left/right = down gear/up gear). The kompressor petrols (C180 and C200) are supercharged and (supposedly) lighter on petrol than you would think. The one I drove seemed a bit hesitant sometimes (but I'm not used to a Turbodiesel).
Manual gearbox is not really an option on these, as the "handbrake" is foot-operated ... it would be impossible to do a hillstart smoothly.
Was quite impressed overall, will be trying a few other cars before we decide though. If we go for a C-Class, we will also try a C200 Kompressor, which I think is a 1.8 litre engine.