I'm also interested in this.
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.
It's not guro. There's some grotesque CGs(computer graphics), but not really any mutilation. What makes it scary is the psychological aspect of being trapped. You empathize with the main character despite their horrible actions. You're dragged right into it with him. You also get to the see the normal world from other character's perspectives, which prevents you from getting used to the main characters distorted view of the world. Don't read the American comic. It totaly misses the point and waters it down. The visual novel is a classic, plus it's sound track really helps set the mood.
Slight story spoilers I guess?
It's really not that bad. The 'nightmare' aspect is mostly just the constant meaty visuals. The MC sees everything as made of gross meaty viscera and the purpose of Say is that she's the only thing that's NOT creepy to him while in reality to anyone that's not him she's probably a creepy meat monster. They never actually show Saya's true form and while there are chunks of flesh in a fridge at one point I don't remember there being any actual on screen violence or gore. There's some non-con and stuff but it's honestly really average as far as the 'fucked up' shit it explores.
That said I still enjoy the game. I haven't read the manga though, didn't know there was one!
It's not a manga, it's a shitty Americanized comic adaptation.