I am a member of both the British Science Fiction Association (BSFA) and the British Fantasy Society (BFS). Readers may be interested in my assessment of how worthwhile they are.
First, some background: the BSFA is the older of the two and is currently celebrating its 50th birthday. The BFS broke away from the BSFA in 1971, primarily to focus on horror and fantasy. However, the distinction between them is not clear-cut: the BSFA includes reviews of works of fantasy, while the BFS reviews science fiction. Both of them are essentially fan clubs, providing publications containing news, book and film reviews, interviews with authors, and so on. More on that later. In addition, the BFS organises a national convention each autumn (the last one was held in central England, in Nottingham), plus occasional "open nights" elsewhere, while the BSFA holds monthly meetings in London, which is fine if you live in London… Both organisations provide annual awards for various categories of publications, with members involved in the selection process.
The BSFA produces six mailings per year of two magazines; Matrix (to become free online in 2008) and Vector. Matrix is the "media magazine", with news, short articles, and reviews of films and TV programmes. Vector is the "critical journal", with articles about books, interviews with authors, and lots of book reviews: in the current issue, more than twenty long reviews and almost as many short ones. In addition, two copies per year of Focus are sent out; this is a magazine for writers, including articles by writers on various aspects of the craft. In the present issue, there is (among other things) guidance on the value of agents plus a list of British ones receptive to SFF works (and those who aren't), a long "Masterclass" by Christopher Priest (on inspiration and observation) and an authoritative article on "Tomorrow's Soldier: The Future of War" although modesty forbids my naming the author…The BSFA also provides support for new writers via a series of Orbiters: "online work-shopping groups where prospective writers can regularly submit their work to gain constructive critiquing from their contemporaries and also contribute their thoughts on the work of others".
The BFS publishes Prism, a quarterly newsletter (in A5 format rather than the A4 of the BSFA mags) which contains news of the society's doings, plus (a nice touch) items from members concerning their latest publications. It contains some general articles, but most of it is filled with reviews; about a dozen of SFF books (including some from small press publishers) and similar numbers of both graphic novels and roleplaying games plus associated fiction. They also occasionally publish (free to members) Dark Horizons, which consists of about a dozen short stories with some author interviews and other articles mixed in, plus other anthologies from time to time.
Both organisations have websites offering news and information, plus an opportunity for feedback and discussion via a blog or forum. The BSFA website is HERE, the BFS site is HERE. The BSFA launched a new site a few weeks ago, which is still a work in progress. At the moment, it is difficult to find much information, for instance about their publications or the annual SF convention, Eastercon. The BFS site was also revamped recently, but is complete and quite informative.
The annual membership fee for each organisation is in the region of £25-30
There is clearly some overlap between the two organisations, but each has its own strengths. In the case of the BSFA it's the support for new writers, plus the greater volume of reviews and articles in a rather more professional style of publication. The BFS offers opportunities for writers to have their short stories or poems published (albeit without payment) and seems to work harder to bring members together, both at the annual convention and also at Open Nights in various locations – not just London.
I joined both organisations about a year ago and, although I haven't attended any of their events, consider them both worth belonging to for the modest cost involved.