I have had some experiences with wiki's before, and I think that they are particularly useful for the classroom and library settings. They can be used to have students or patrons discuss different topics, books, or make recommendations to each other. It allows everyone to have a say, and say what they would like to say in an arena that is dedicated to a particular topic.
My school is doing a book study currently, and made a wiki for the discussions. I think that this was a very good use of a wiki, because anyone and everyone can go in and post their thoughts, or create new discussions based on the book.
The nice part about Wiki's is that everyone can participate, which differs from blogs, where there is one participant, while others can comment.