If people were expecting a manga about three overpowered high school girls in a military mission to be realistic, I don't really know what to say. What this manga does really well, though, is giving a somewhat consistent hard sci-fi feeling that helps building a strong wall of suspension of disbelief. It's not a case that every objection, that previous reviewers have done, has been actually explained or explored over the course of the story. Starting from Ibara's "naivety", that is shown to have way deeper roots than our usual Mary Sue. Apart from some silly moments (and what long-runner doesn't have them anyway), the plot stays consistent and increasingly engrossing. The author often uses last minute saves, but it doesn't feel annoying, because he goes out of his way to build them up, so that they don't feel like coming out of nowhere. One glaring example may be a huge hole in a zone of Tokyo, which is obviously presented to us because it will be needed later: it's much appreciated, because the reader does know of this hole when it finally comes into play, and the fact that it doesn't happen immediately after it's been introduced is also a plus. The characters are a strong point of the manga: we're not treated to long expositions about them, instead their personalities are shown through their actions, which is typical of good storytelling. It's great to see that even the survivors, with little screentime, are given depth: it's not often that I actually care about the annoying kid who puts everyone in trouble all the time, or that I actively root for the characters to survive. Character development is well done most of the time, but we do get some logical jumps for some of the characters. Drama and comedy are mixed with the right timing, and apart from a couple of moments that focus on action for too long, the pacing is handled in the right way. The only major flaw is the excessive parody of the countries' leaders, which feels out of place in respect to the realistic feeling the manga is trying to give every other time. The manga is never rushed and also wraps itself up quite well, even if a bit of fridge logic makes it clear that some other chapters would have been required to truly end the story.
All in all, it's not the best manga I've ever read, but I found it to be above average, and while 7 is the mark I think it deserves, that's based on my way of giving votes: according to this site standards, it's very underrated.