I was a member of the Science Fiction Book Club for a very long time (probably over a decade). I joined back in the pre internet days (in the early 90's). Early on I thought it was just great, I would keep up with the mailings (you had to explicitly say you didn't want the book(s) of the month in order for them to NOT automatically mail it to you) and felt like it was a pretty good deal (although with shipping I was always a little bit unsure).

But after I got somewhat of a life (and forgot to fill out those cards saying I didn't want the book of the month on a monthly basis) they started bombarding me with books (which I always sent back), but it was a hassle (and I'm sure my mail person hated me). I did eventually get on the list so that they no longer automatically sent me the books, but by then I was done with them.

To me the web (Amazon, B&N online, etc.) and the proliferation of huge physical book stores totally obsoleted the need for the SFBC, especially in the age of cheap shipping. Also, I agree with some of the other posters, your library is probably a good first stop, that's were I got started back before I had money to burn on books.