Maybe I've desensitised myself a little by occasional forays into the genre, because it has somehow turned out not to be as creepy as I had expected, but it was still creepy.
The creepiest parts are the first third of the book before the characters start realising what they unleashed (while the readers have more knowledge, or at least suspicions, than they) and, of course, the ending.
In between and throughout the book there was, however, plenty of gore which I tried not to imagine too vividly.
The story overall didn't offer anything new regarding concepts, but it was nevertheless compelling and well-written, and it definitely delivered enough material for a nightmare or two.
All in all, I now feel weirdly accomplished for having read it. ;)