Hmm… that wasn't bad at all. Totally style over substance though… but in a Lovecraftian way, not in a "no merits but shock value" way.
It was more fascinating than scary. I think the Through the Looking Glass feel of everything, including characters, not feeling real did away with all the existential dread it was meant to evoke.
Visually, it was fascinating, but nothing groundbreaking. In terms of narrative structure, ignoring all the filler in the middle, it felt fairly vanilla.
I don't see myself revisiting it, but I would watch an animated adaptation out of curiosity.
All considered, I would recommend it if you have nothing better to read, but I wouldn't label it an essential manga, or horror, reading.