I do participate in Early Reviewers. Since I joined in April of this year I have requested books from every batch and have been lucky enough to receive three: Woman of A Thousand Secrets by Barbara Wood, The Matchmaker of Perigord by Julia Stuart and The Victoria Vanishes by Christopher Fowler.
Out of the first two I would have to say my favorite was Woman of a Thousand Secrets, but I liked both of them. I just received The Victoria Vanishes from this month's batch, so it's too early to say about that one. I try to request books that sound interesting to me and I think the algorithm does it's thing well, too, matching me up with books that fit with my library.
I do participate in the discussion group but not to the extent that I should. I have gotten very busy with other reviews here on my blog and that cuts into my time a little bit. It is a great program and I really enjoy being a part of it.