Plot/Story
The story is action-filled and interesting. It was very engaging, to the point where I'd sit down to read a few chapters and look up what felt like minutes later, only to realized that I'd been reading for an hour. The plot is mostly unpredictable, with good foreshadowing. The series is one of those where the characters and audience start out not kowing anything, and then it reveals everything over the course of the story. I really, really liked it.
Characters
The characters were good as well, with, other than the protagonist and a few others, very little plot armor. They also suffer trauma from their experiences instead of just moving on like nothing happened, which is something I appreciated. The enemies they face are also unique, though a bit underdeveloped.
Drawing Style
The drawing style was well done and the characters and enemies all looked unique.
Enjoyment
I really, really liked it! It ended well, in my opinion, but I still wanted more anyway!
Overall
You can probably tell already, but I really enjoyed it. I would recommend it to people who like "Attack on Titan" or "High school of the Dead" because of a similar overall feel and apocalyptic setting.