Just finished the last episode. A very mediocre show overall. Not sure if I was expecting anything else though.
I didn't really pay attention to the whole Mana/Gai/Shu/Inori story. I'm not such a big fan of plots where the fate of the mankind is somehow tied to some drama between only a few characters. Besides all the characters were so terribly hollow that it was just plain boring to watch. None of it really stepped out of the comfort zone. Perhaps this is just a stupid generalization where I try to apply my personal preferences to everyone else but I claim that all the character development fit perfectly the coordinates of your basic action hero fantasy. You even had this tragic but none the less conformist moment of collapse into evil where Shu abandoned his egalitarian ideals and adopted the position of a cruel leader I'm saying conformist because it cleared up quickly that the basic function of this plot twist was just to turn him even more epic: he could then conveniently play this game of heroic sacrifice towards the end of the series. I'm more and more convinced that if you want to find well written characters try looking for stories where they're ambiguous, unpredictable and/or just plain unlikable.
As usual, the most interesting stuff happened on the background. I really did like the thematics of evolution/selection vs. (what could be called) humanist idealism. The most interesting part of the series was the school siege part where the students had to come up with ways to survive and organize their little community. Some interesting experimentation with the thematics there. That being said, I don't think this theme was developed to its full potential either. Ironically evolution appeared as a sort of metaphysical, almost religious entity here, and this basic animism sort of ruins any serious attempts to get something out of the concept. I would rather say the conflict was between hierarchy and egalitarianism. I'm glad the latter won