My New Year's week project was getting my books cataloged online. I couldn't decide where I wanted to set them up, and in the end, I doubled up - I have libraries on both GoodReads and
LibraryThing now. (Shelfari didn't make the cut.) I also have all of my basic book info (ISBN, author, title) in an Excel spreadsheet, since both sites will allow you to import book data that way and it's much faster than entering books one at a time!
I'm going to keep the dual system going for awhile till I decide which I prefer, but right now I suspect that LibraryThing will win out in the long run.
If you have a library account on either or both of these sites, please let me know or look me up so we can connect!