Library Thing was definitely easy to use. Once I went through the site tour, it only took me a few minutes to set up the account. I just grabbed a few books and typed in the ISBN. It took no time at all. I added a few tags, just really random thoughts about each book. I'll need to use tags a bit more to get used to them.
I can see where Library Thing would be very useful for people who spend time on their own book collections. I will need to spend more time with it to see what other features are offered by the site.