Manhole is what happens when you mix detective stories, Japanese virus flicks, and disturbing imagery. It opens fairly dark, bleak, and weird. The story flows nicely, and doesn't hit any snags along the ways. Pretty soon after the opening, everything goes absolutely nuts, but not so much as to where you cannot understand a single thing. The art design isn't bad. On the level of gore, it has appropriately enough for the given story and context. It does have a major cringe factor, which you see quite a few times, and one scene in particular just makes my skin crawl to the nearest bottle of sanitizer. One problem for me is, it's too short. I know mangas are supposed to be of sensible length so that the story doesn't keel over, but I finished it in 3 days. It would have been 2 if I didn't have classes to get to.
Overall, worth a read, but it will probably be a manga you won't revisit.