This anime is fantastic. The most amazing thing about it (especially in the second season) is its unflinching attitude to the darker themes - some scenes will shock you, others will make you cry, and all of them are fantastic. Just when you are about to predict the story it will smack you across the face and tell you to try again. When you know that almost 95% of all the characters can either die or get some seriously messed up treatment, it makes for some of the most tense TV moments... ever.
The characters are generally good. You'll find yourself picking sides and favourites very early on due to the nature of the show. The show presents all sides, all characters and their motivations while managing to keep the some huge story elements secret until they are revealed - that's good writing and it engages the audience.
Animation is literally eye candy. I don't normally enjoy CGI effects, but this show perfectly marries 3D and 2D animation into one. The production values of this show are amazing and I applaud the animators and artists for not taking a half assed approach to some scenes while giving special treatment to others. The OST is used well and the sound is very nicely done.
Overall, 9/10. The 1 deducted was due to the story maybe introducing a little too much characters early on, making it fairly hard to keep track of in the first few episodes.