The series starts off with cyberpunk dystopian overtones and fades into dark fantasy as episodes go by. It is really character driven, so the emphasis is on the main protagonists' personal evolution as the world around them changes, the two main points being finding one's self and madness as a reaction to those changes. Unfortunately, the plot somewhat suffers from that; it gets really confusing in the last few episodes and requires multiple views to really understand what's going on. On the other hand, that leaves quite a bit to the imagination of the viewer, so it's kind of an open story with many statements and less interpretation. Some interesting subplot stories aren't developed, which is a shame because the ideas are really interesting. It also sometimes feels like the writers have thrown in allegories and spiritual or philosophical imagery just for the sake of it, without really expanding or incorporating them into the main storyline. In spite of that it's still one of the better anime shows out there, and has the feel of a graphic and futuristic essay on the self and the relationship of the self with the world.
