One of the most well done animes out there. Nicely animated, deep story and awesome character development. A definite must have, though not if you are only checking it out because you heard its hentai/porn, or for bloody fight scenes.
But be warned – don't watch this while depressed. In fact, don't watch it unless you find yourself too high one day and wants to feel miserable. It's dark and utterly depressing.
Nevertheless, nice film, and for all that I've said, watch it one day.