I'm going to break up what I have to say into 5 paragraphs.
Story: Interesting story, unique, but be prepared to be almost completely lost for the most of it. It's done in little segments that, when viewed all together, tell different parts/versions of the same story. Hang in there, it is worth the watch.
Art: You won't see much in the way of fancy graphics here. Everything will be fairly standard. One tone of shadow, nothing really mind blowing. That said the contrast between its goofy, almost innocent style of animation mixed with its darker scenes was really quite the contrast, and gave things an edge I really enjoyed.
Characters: Each character is fairly unique, different personality types with their own back-stories and motivations.... that said, why they do things don't always make sense, at least not to me. I won't spoil anything, so I'll point out the one thing that stood out the most to me without giving away any plot points. Shion has a gun... its in plain sight basically all the time, and it fits into the back story, thats all fine. BUT SHE NEVER USES IT! No one is ever like "Hey, you're sixteen, why do you have a gun?" Or "Hey, that would be really useful to use right now." Which trust me, there are plenty of times it would have come in handy... but nope, it just sits there, every episode... taunting you with its uselessness. And at quite a few points in the story you are going to be like "Oh he is definitely going to confront them about this... what?... he's just going to be passive aggressive and bitchy? They just tried to kill you!" That said, the rest of the story makes up for this.
Enjoyment: If you don't mind gore and enjoy a dark story, then this is a good anime for you. If however you don't like to see violence against children in your fiction... well, I'd avoid it. The characters motivation make enough sense that it is watchable, and the story is solid. You could definitely do worse.
