King Crimson has had a large enough cult following to keep them successful for the 30+ years they've been active, but otherwise you won't hear much about them. They tend to be experimental, so you may love one song and hate the next.
Sleepless, Frame By Frame, Discipline, Another Red Nightmare, and Starless are a few of my all-time favorites.
Sleepless (in concert):
http://www.youtube.com/watch?v=5TTCk4rKnMM