This manga is interesting, but not without its flaws.
Story: 6.5
It's a mix of the zombie and dark magical girl tropes, but has some nice redeeming qualities. Unlike a lot of zombie/disaster stories, the characters often survive because of their wits and good planning rather than dumb luck, which is refreshing. The story becomes more intriguing as it progresses and takes some interesting and unexpected turns. The trials and mysteries the characters face become increasingly engrossing the more you read. Also, the pacing is pretty on-point.
Characters: 6.5
The main character, Kogami, is pretty bland. In fact, he's the only character among the main cast whose name I couldn't recall. My favorite character is probably Yoruka. She seems like a typical "oppai" character, but as she develops she becomes very charming and more well-rounded. She's got a good mix of strengths and weaknesses - she's pretty tough and doesn't take shit from anyone, but has her fair share of insecurities. I find Akuta (the police officer) strangely loveable despite his psychopathic antics. Normally I'm very turned off by misogynistic characters like this, but his ability to scheme and think on his toes and his strange dedication to Yoruka (which seems to take a step beyond regular sexual attraction in later chapters) made me like him sort of begrudgingly. Tsukune is a little trope-y, though her character development was well-paced and her "twist" fairly well-executed, if a little melodramatic in some aspects. Sayano is also somewhat of an archetype, but in an unexpected way.
Art: 6.5
The mahou shoujo look awesome, but other than that the art can be lackluster at times. The gore, while pretty damn cool and inventive in some parts, can also be confusing to look at. The anatomy also needs work.
Enjoyment: 8.5
Despite its flaws, I really enjoyed this manga. It has a strange charm that makes it sort of addicting.
Overall the people who rated it a 1 are probably justified, but I would say give it a shot if you like the genre.
Also: I feel it is of note that this story has some misogynistic elements and several instances of sexual assault (though none of the characters actually get raped).