Yes, Adultfriendfinders, or AFF is real. I know a few people that have used it and found some great "friends with benefits."
As I mentioned before, screen carefully, make sure you state your exact intentions. Once you find somone of possible interest, exchange a few emails, get some "real" pictures, and a real email address. There are a lot of people that just post pictures off the net or try to get you to view their website for a fee. Also, some people just do it to see what kind of weirdos they will get.