I feel like lots of reviews here are so very low because many people were disappointed with what ZNT became, in contrast to what it would have become. If I were to say I wasn't disappointed on how the series turned out, I would be lying.
When at first, the two protagonists steal a nuclear reactor thingy (we didn't know what it was at the time), and subsequently destroy a building, I thought this series was going to be the next masterpiece, the next Madoka. A thought-provoking, edgy, dark tale that poses various moral dilemmas to the viewer. I could almost see how the story would turn out; as their terrorism continues, they accidentally murder civilians. They begin to question if what they are doing isn't simply fueling a circle of hatred, and are subsequently driven insane. It would have been such a juicy series to watch. Alas, the series chose to make it a simple tale of "solve the mystery!". There was no questioning of morals, and no questions were posed to the viewer. I was so very disappointed.
But this review is about what the anime IS, no what it could have been.
Art: 10/10. The art was beautiful. I found myself admiring some of the art while watching the anime, which was, honestly, a first for me. Some of the scenes were like animated canvas paintings. The art group for this series needs an award.
Story: The introduction of 6 did derail the story quite a lot. No longer are we focused on the plans of Nine and Twelve, but now our protagonists are trying to stop Five instead. But what I found most annoying was the bullshit the show pulled when it comes to Five. Yeah, if America pressures Japan, most things will go their way, but I simply don't believe that the Japanese government will just allow a train full of civilians to be blown up. Same for the destruction of a commercial plane. "Let's blow up a plane to catch two terrorists". Give me a break, that'll never be approved. These plotholes were pretty annoying. Otherwise, the pacing was slow, and the "mystery" was pretty meh when it was revealed.
Characters: I feel like the series screwed up with the characters as much as it did with the story.
Twelve would've been so badass if they had more fighting scenes. It was implied he was super athletic, so why not make him do some crazy stunts? Also, I forget why, but he changes his mind about the "terrorism" SO quickly. I think he starts moaning about it in episode 3. Like jesus, you spent 10+ years thinking about the plans, and you think you should quit now because it's "wrong"? What???
Five is a psychopath, but the series tries to make us feel kinda sorry for her when she dies. But honestly, I don't know how I was supposed to feel sorry for a person that tried to murder hundreds, TWICE.
Nine was silent and calm, but I feel like they screwed up on his delivery, too. He spoke so little, and we were given so little access to his thoughts, that at the end of the series, I was left with a tiny but more knowledge about him than from episode 1.
The detective was the best character by far. He was entertaining, he acted like a human, and he was great. Loved him. I also loved his chubby sidekick too.
In general, they tried to switch perspectives between characters throughout the series, which might have worked, but this series is only 11 episodes. Because of this, viewers were left with a little bit of knowledge of a variety of characters, but not much about any single one. The fact that they don't talk their minds much didn't help, too.
Overall: They hyped me up for a dark, morally ambiguous story, and I got a straightforward, catch the bad guy story. The story wasn't bad, but it wasn't great, either. Viewers were given so little information about characters, it was frustrating. I would've given it a 8, but the plot holes really bugged me. Overall, I would recommend for anyone to watch it, as it isn't a bad story, the art is fantastic, and the music is hauntingly great. But don't expect anything beyond a "pretty good anime".