Being a book lover, I have looked at LibraryThing, Shelfari and Goodreads. I don't really use any of them, though I have attempted to use LibraryThing in the past. I found it pretty overwhelming - and I used to be a cataloger. The only thing I would use one of these sites for is to make a list of books I want to read since I come across about 5 a day, but I already use Amazon for this. I think Shelfari is more user-friendly.
Here's the link to my LibraryThing catalog: