I like the BMWCCA but as a forum for the E39 M5 it is basically dead. If you need information about the E39 M5 check out M5board.com (no affiliation) http://www.m5board.com/vbulletin/e39-m5-e52-z8-discussion/
If you frequent or post on the m5board I think you're affiliated! It is a good forum, even for E28 info.
Good point I guess I bear no affiliation to any entity that derives financial compensation from said forum