I have never joined any of them and think of those as very amateurish but that's only a feeling I have, as I have no experience with it but I have always had the feeling to stay away from it and I tend to trust my intuition.
I just began experimenting with surferswap. I put an iframe on a low income site with links to to other sites. When someone clicks on one of the links, I get a credit and my link starts appearing on other people's iframes. I made the link point to a potentially high income site. Because the links are in iframes, I don't think Google will ever index them (probably good since most are unrelated to my site).