NOTE: I watched this series in English and season 1 only.
Series in one sentence:
A boy with no real talents and strength is lucky enough to have a hidden power that saves his ass.
Series in more sentences:
In a world where giant and mysterious human-like monsters roam the earth and eat people, a young boy is determined to join the army and fight against these creatures.
I was never sure what to think of this. It popped up everywhere on the internet, but I was never motivated to check it out, it looked painfully silly to me. But the setting makes it work.
Still, I'm more amazed that the series' biggest climax (the main character's power) was never spoiled to me. I was surprised to be.. surprised. I expected a generic war cartoon without much else going on.
Even so, the confirmed enemy at the end was rather lame, as this certain character never played a big role and the main character never had a relationship with them; which is why I didn't get his hesitation to kick their asses.
Speaking of which, one big flaw the main character has is that he needs to get motivated every time again in order to do something, and for a boy who claims to be motivated, that doesn't make sense.
Other than that, awesome series, there's not much to complain about. Watch the English version if you can, it's very well done.