Story: I really liked the concept and pacing of the series. It didn't go too fast but not too slow either. It usually consisted of 2 episode long stories with some events in each that dealt with the bigger picture. However, I am disappointed with the ending. I felt there were too many things left unanswered and vague. The explanation for what happened in the past was also unclear. The plot did have some predictable moments but there were more surprises to balance it out. Although for the most part, I really enjoyed the series. It was pretty dark but it always had a light side to balance it such as the decetive and the humour. But it is definitely a more mature series with it's content and violence.
Animation: Crisp and clear. I loved the animation. The movements were fluid and dynamic. The scenery was detailed and vivid. I also liked the character designs which had little touches to make them stand out. The openings and the first ending were memorable. The second ending fell flat a bit. However, if you want to watch a nice action series, this is definitly one to watch.
Sound: For the most part, the soundtrack was really nice. I would like to go out and buy it. There are a few pieces that are less than stellar though. The openings and first ending were memorable but not the second ending. The seiyuus did a great job especially with Hei. He went from calm Li-kun to the black reaper remarkably smoothly. None of the voices were annoying even the pink haired decetive assistant.
Characters; I liked the characters especially the contracters. I felt the humans were less likable for some reason. I would have liked more of a background of Hei and some of the other main characters. They did give some background detail but it felt a little vague and incomplete. I don't completely understand what happened with Hei in the past or with Yin and Mao. However, the cast is not made of shallow characters. They have depth and are more than what they appear without being predictable.
Overall: A nice change of pace series with action and drama that are bit darker than most. It can be really sad but there is enough humour to balance it out. However, expect a disappointing ending. Good news is that there is a second season so it has chances to improve. It's definitely not a series I'll easily forget.