This is one of those mature mystery type of mangas , replete with monsters , hidden conspiracies and the like.There is a lot of action , good characters and a intriguing story line. It can be a bit gory at times as well.
Like the previous reviewer said ,it does have similarities to Gantz and HOTD and falls into that type of genre.
I am enjoying it and if you liker the other two , you will probably like this as well.