I actually ordered the manga and read the two available volumes recently (sadly it's on hiatus in Japan), and I have to say it wasn't even nearly as intriguing as the visual novel. It doesn't really have the same kind of creepy atmosphere that the game delivers and it's going at a much faster pace, which almost made me feel uncomfortable at times. I think you would find it much more enjoyable if you played the game first tho.
I actually ordered the manga and read the two available volumes recently (sadly it's on hiatus in Japan), and I have to say it wasn't even nearly as intriguing as the visual novel. It doesn't really have the same kind of creepy atmosphere that the game delivers and it's going at a much faster pace, which almost made me feel uncomfortable at times. I think you would find it much more enjoyable if you played the game first tho.