I just took my car to RMP this week to have my M3 CAB's and IE subframe bushings installed. Place was clean, both guys (Rocco and Mark?) were friendly, and they did an excellent job on my car - could not have been happier with the service I got.
It sounds like you can't really go wrong with either RMP or Bimmersport. I went with RMP because I called both, and RMP just happened to be cheaper.