I just happened to come upon this anime by chance, and I'm very glad I did! The characters are fascinating, the story intriquing and the themes very philosophical and thought-provoking.
It is quite dark (a good thing, in my opinion), but also laid-back and comical in many moments. Highly recommended!
(I would also recommend that you check out the original manga. It actually has some different story-lines than the anime, and is just as great! :D)