I have finished the "game". It lasts for about 6-8 hours, depending on how fast you read, and only has 2 choices in the entire playthrough.

The "nightmarish" aspect depends on the audience. Normal people would probably consider it disturbing, particularly the parts that deal with rape or murder, but, for people who like guro, that's pretty much just a normal day, isn't it?

The plot is interesting, and the narration and art are good. The sex scenes, however, are rather lackluster.

I haven't read the manga, and, from what I've heard, it isn't that good.