There's something of substance in the premise here but that substance is greatly underutilized. There is a tremendous lack of explanation which makes everything feels random. The plot jumps around randomly pulling out plot points without properly developing them and hence having everything feeling rushed without actually giving a feeling of any structure or solid development. A good example of this is the ending, where they seem to pull an attempt at making the villains sympathetic, despite discussing how they massacred a heroine's family only a chapter or so before, and hence this fails to work at all. The characters too seem to have interesting personalities and traits in general, but the manga doesn't really seem to develop that all that well at all and the unique traits of all the characters don't really shine through. The motivations for most characters, especially Zonmi, feel tremendously underdeveloped. And in the end, beyond possibly Manami, most of them feel quite forgettable. The art is pretty good. 

tl;dr: A relatively interesting premise and characters but one that tries to push forward without properly developing anything at all.