Well, I tried this as my 2nd VN (after 999) based on all the recommendations and can't say I'm too impressed.
Warning - SPOILERS!
I'll start with the bad.
Some things don't make sense and break the character concepts. Saya basically absorbed human culture, making her hungry for "love" from all the romance novels. I'm OK with that. But why didn't she absorb even more basic things like compassion (which is a basic component of contemporary human culture - hero saving the world/girl/kitten and all that)? She enjoys torturing and driving humans insane (Yoh) - how does this makes sense, based on her premise and falling in love with a human?
There are no sympathetic characters except Yoh (which is only because she gets such a cruel fate, not because of her actual character). They all have utterly selfish, simple motivations. This wouldn't be so bad in another novel, but here the story's impact (Romeo and Juliet, warped) relies on you feeling sympathy for Saya and Fuminori - which you can't, as they are both sadistic assholes. "Everyone is an asshole" needs these assholes to be at least interesting, but no time here is spent on character development and some feel like cardboard cutouts (especially Kouji).
The story itself adds nothing to the genre. I regard Lovecraft as a mediocre writer with great ideas that had tremendous influence. Unfortunately, instead of trying to add some interesting spin to the ideas, it just takes the worst of his writing. "Unspeakable, impossible horrors that drive you insane" is a bit of a weak and somewhat overused concept, more of a cop-out for actually setting up the scene to be horrifying in itself. It's fascinating the first time you see it, but it's not scary or interesting anymore (not that it ever was scary).
The porn. Holy shit is it more disturbing than the rest of it combined, and not because of the premise, but of how bad it is. Giant pixelated cocks and "cum in me please" stuff that don't belong in something aiming for more than just a hentai game (yeah, I know Saya wants his cum - but couldn't you convey in a more tasteful way?).
The good:
It's very short and fun while it lasts - you want to see how it ends, after all. I read it in 2 evenings and only put it down once I've seen all endings. It won't leave any lasting impression, but I guess it never aimed to do so.
Some memorable scenes, like Oumi's death and artist's insanity - props to the graphic artist.
The pacing and the structure are nice and clean. I liked the Mad ending which was a throwback to the traditional Lovecraft endings that I loved as a kid. Other endings are also decent, like the Bad one - reminded me of the Colour Out of Space, somehow.
Overall, I'd recommend if you're feeling an itch for a Japan-inspired Lovecraftian story, or something to kill time with. If you're looking for something of lasting value or some new spin on the horror genre, it's not that good. (Bear in mind, I don't know about the general quality of VNs - maybe it's a masterpiece among their stock, maybe not).