I watched The Butterfly Effect last night. I like movies that keep me guessing and "on the edge of my seat." The Butterfly Effect did both of those things. I wouldn't recommend it for everyone: It's intense, with some rather violent scenes. But, if you're in the mood for a "mind game" movie, check it out.
By the way, most of the critics panned this movie, but I'm not one to listen to the critics! ;)