You may want to check the following link and see "Sponsors" on the left side of the page. I just ended my warranty period so I haven't got a good indy shop recommendation. If you want to work with a dealer, Schomp BMW is a good choice. Good luck.
What location is the car at? If it north/nw of Denver, then Bimmerhaus is probably the best one. If it is south/sw then Absolute will be the one. I have taken my M5 for service at Absolute and I do like and trust them. I have met the folks at Bimmerhaus at track events and they know their stuff but have never taken the M5 there as they are too far from where I live. PM me if you want me to go take a look at the M5 (within a reasonable distance) and tell you my personal opinion.