I was pleased to see this one. At first it seems dull and boring - because we come for this cover. We want a horror movie. But that's not what happens. And then it gets interesting... We want to know more about it - while feeling guilty for this very desire. Then it goes completely overboard with its intent; and we are shocked. Grossed out, for many of the things shown. But we can't stop looking. We can't turn off the movie, because we want to know what happened. This movie engages the audience with a simple and subtle - yet incredibly strong - grip.
The end brings a terrible, painful surprise. We struggle with what we see. Though some of it might seem far fetched, since we're in its grip, we can buy the premise. I think that the director could've served the third act with a little less exposition; much of the ending would have a lot more impact if there was no character narrating it. But the movie still delivers a good story.
In some aspects. it reminded me of Oldboy.