The tale is too long to go into here, but basically my beef is threefold:
The general unreliability of the thing (check engine light on four times in three years of ownership; I had a Renault Grand Scenic previously, which was faultless)
The incompetence of the service staff at MB, who it seems couldn't find their arse with both hands
The ridiculous prices you have to pay for such inept service (e.g £220 "investigation fee" every time that light comes on, £220 for an air intake hose, not including fitting... My mate had an old Saab and to change the air intake hose on that is £15 for the part, and you can do it yourself in half an hour).
I'm sure Mercs were great... Once.