I don't like LibraryThing. It was too hard for me to navigate and there just seemed to be so much going on that I was left confused.
When I try to use Shelfari, I have to shut off Peerblock because it just doesn't like the site. But Shelfari is good because if you update character profiles and add trivia stuff, it all shows up linked to your book on Amazon. So Shelfari is a good tool for making sure people are notified about all the little extras you've thrown into your book.
Shelfari is pretty user friendly too, though I like the tags on Goodreads a lot better. I can have more shelves, and Shelfari just crams everything together. Plus, it doesn't remember my settings
Still, very useful.