After cruising around the three shelf sites I couldn't resist
Shelfari's interface. Like my students, I like to see a book's cover! No-no or not, we often choose (or are initially drawn to) books because of their cover art. Or are too put off by cover art to open the book.
Shelfari makes it easy to see who is reading or has read what I'm interested in, to find alternate reviews, and get a feel for how titles are being received. An interesting addition to the tools I currently use to make choices - School Library Journal, Hornbook, Booklist, Bulletin - but what weight to give it? I think there is a possibility that, over time and assuming due diligence, this might be a more personal and trustworthy resource.
And it's fun!