People who enjoyed Paprika should try to get ahold of Mind Game, directed by Masaaki Yuasa. Psychedelic Gleam already recommended it in this thread, but damn is that a crazy movie! Really a masterpiece of animation and very intense.
Its been a while since I've seen this one. Although I still love it, I realize now that it has been done before MUCH more artistically, at least in the world of anime.