I'm a member of 3 sites currently : SLS, Bisexualplayground.com, and Swingersdateclub.com
I've joined others in the past, but decided to stick with these three. The reason I chose these were because of either great message boards (like this one!) or because of the extensive profile set-up.
Also, it's difficult to determine long-term success with the websites. I've found that many members will only look at the new/updated member profiles. Which is fairly understandable because if you take the time to do a search for whom you'll be compatible, then the only additional folks you'll meet are the new members. So for the first few days/weeks of joining, you'll receive a flood of emails, then .... nothing.
Oh, and I'll usually check my messages every few days, just like my regular home email. The great ones are the kind that notify you via email if you have a message. Therefore, you won't waste your time going just to check.