Just purchased a 156 and though it would be a good idea to join this forum.
Old car was a 2004 Merc C270 CDI with sports pack. Had to send it back when I left UK as it was on finance. I was looking for something a bit different (and less expensive) and settled on a 156 JTD 20v Mjet sportwagon (2004 face lift)
Main observations between the 2 cars:
Better build quality
More "sorted" handling
A bit boring
yahoooo engine, feels a lot quicker than the merc (although the book says it has 2 ps less (175 vs 177)
Handling is a bit vague round corners and the suspension doesn't like pot holes.
Looks better from the front than the merc
sounds better than the merc (actually you could hardly hear the engine in the merc when inside)
All in all a good choice as a replacement for the c class and very good value for money.