I'd seen Library Thing demoed previously. Obviously it is designed for a social networking group. If you are a private person and don't want what you own up on the web, this is obviously not for you. The "how many have it" and the tag clouds that are created can be a plus.
The big negative is not being able to put up things that aren't books. There are a few scores that have been input but there is no way to say what kind of score something is. This is extremely important for musicians.
Sound recordings and videos are not really represented.
There are now ads on the pages and links to amazon, abebooks, etc. Again, not necessarily bad, but they are ads. I don't know if the ads would show up if you paid the $10.00 per year for adding more than, I think, 200 books.
One advantage is that this is webbased, so you can access stuff from anywhere and are not dependent on something like a spread sheet (I've got many of my books on an excel spreadsheet) that is loaded on some type of drive.