I use Good Reads now for my personal use but I could see that it would be a great place to review books for purchasing.  I haven't really used Shelfari but I think it would be another great place for book reviews.  The widget that they talked about so that people could see what new books your library has purchased would be awesome.  I also like Doris Ann's comment of using the cover art from Goodreads to put a newly purchased book onto facebook.