I agree with Mykle's review: the book is fine, but the movie is a clear improvement. The story flowed better, and I thought all the important changes to the story were good ideas. In particular, the romance with Katie was more fun, and so was the ending.
Above all, though, the film medium allowed many sight gags that were impossible in comic-strip form. Starting with Hit-Girl's amazing balletic routines as she dispatches dozens of huge, heavily-armed opponents in hilariously bloody fashion. They made the movie for me, and in print form they're just hinted at. You may argue that that's more subtle, but, trust me, subtlety is not what Kick-Ass
Three stars for the book, at least four for the film!