When I was shopping for cars last year, I really wanted to get a 2003 540 sport. Beautiful cars, fantastic performance, practical, etc etc etc. I eventually decided against it due to reading in reviews/ratings and in nearly every forum I checked that the 540 could wreck you due to maintenance issues. The impression I got was that the car was not exactly dependable, even by German standards. Take that for what it's worth--I've never owned one.
My sister on the other hand used to drive an old E34 540, and you could depend on it every week to be an absolute shit pile. We once had to pull over on the way to school because, literally, a small fire hard started in the engine bay.