In the past I was a member of the BSFA, which is primarily a fan group with publications including reviews, news, etc., but as I got busier as a writer I let my membership lapse - I simply found that I never seemed to have time to read their publications or attend meetings - there always seemed to be something more important to do. I joined the SFF when it was first formed - it's basically an educational foundation which publishes a quarterly review of papers on various SF-related topics. I've continued to support the Foundation since from time to time I've found it a useful source of information, and have donated books and magazines occasionally.
My impression, after thinking it over, is that I'll probably keep going the way I am - my interests tend to the scholastic, and I get my fannish fix from livejournal, newsgroups, and conventions. I'm not knocking the BSFA, it just seems a little less useful to me and relevant to my main areas of interest.
Incidentally, the final item of the day was a panel organized by major_clanger, who is pursuing a career as an Intellectual Property lawyer, the other panellists being me and John Clute (SF Encyclopaedia etc.) discussing copyright problems related to SF and fandom including fanfic, the recent Dr. Who knitting pattern case, the Star Wars stormtrooper costume case, and the Harry Potter Lexicon thing. Some interesting stuff came out of it, though at the moment it's probably fair to say that nobody is quite sure which way the courts are going to go on most of it. I hope that major_clanger will eventually put a summary on his livejournal; I'm not going to pre-empt him on this, since I would really hate to get things wrong.
In all an interesting day, and a nice break from routine.