From the animation to the characters and especially the soundtrack, this is literally a "must see before you die" movie. As with most Studio Madhouse productions, the quality is excellent.
The story is unique and refreshing. The characters aren't as fleshed out as they could have been, but the action makes up for the sometimes lazy writing. This particular anime almost sent Madhouse into receivership. The director was let go after it was finished due to the tremendous cost overruns involved. But the end result is genius.
Some nudity, and a little fan service, but completely worth a watch. It's on YouTube in HD for free, so there's no excuse for passing it up.
Lastly. Crank up the volume for the soundtrack. You won't regret it. :-)