Shelfari is easy to use and it just looks cool. I also enjoy that Shelfari includes when you read each book with your review. Sometimes it is nice to look back and see when you read something.
All four sites allow you to make bookshelf widgets to display on your blog.
All four connect to Amazon for easy book purchases.
WeRead and Shelfari both connect to Facebook.
Librarything and Shelfari allow you to "tag" books.
No matter which site you choose to use I think personal cataloging is a great idea. It gives a great outlet for discussion and gathering of new ideas. Students who wouldn't necessarily recommend a book in front of the class might be more comfortable posting one to a group bookshelf. There is something very gratifying about looking back over a shelf and knowing you have read everything on it.
Now if only they will make one that connects to my Kindle and allows me to write a review right when I finish . . .