By and large, I liked the 23 Things experience. It was a little overwhelming to sign up for or sample so many web services in a constant stream, but a lot of interesting "Things" were presented.
I liked the image generator and the motivational poster that I made, simply because that's the one that I had the most fun with. I liked Youtube, because I felt that that was the one with the simplest search engine to get the subject that you wanted. Although, admittedly, that might be because that was the one site that I was already most familiar with before it was assigned.
Finally, the one that appeals to me the most in a library setting is LibraryThing. It has the most comprehensive amount of information geared to and for book lovers. And, ultimately, that's the number one patron for a library.