I am utterly addicted to this! It's like MySpace/LJ/Facebook for booknerdz. :)
Now when people ask about my favorite books and authors, all I have to do is point them thataway! I've added about a hundred of my favorite books already. I'm probably going to buy a paid account...it's only $10 a year, and you can have as many books listed as you want (the free account has a cap of 200).
But my favorite part are the recommendations! Just click on a link and it'll show you the most popular books listed by people who list many of the same books as you. It can even filter out books by authors already on your list. You can also have a watch list to see what other users are reading.
You can also read and write reviews and organize your "shelves" with tags. The signup is free and easy, you just enter a username and password.