I intentionally went into this book without reading the back or anything about it - all I knew was that my co-workers said it was scary. One of the original tag lines for this book was that you would be scared but you wouldn't know why, and I actually agree with that assessment. There is a layer of unease and tension that lies over every scene, especially in the first two thirds. I just felt creeped out, even when nothing frightening was going on. Honestly, for me, the experience of reading this was a lot like having anxiety - you're unnerved and anxious even though there is no direct reason to feel that way. When it comes to atmosphere and tone this book nailed it.
A book is more than atmosphere though, and here is where I have to talk around things (otherwise I would spoil the entire book). The book hinges on a twist, which I think is pretty evident when you open with a loaded statement like, "I'm thinking of ending things." It sets you up to wait for that moment, and look for the turn. That's a big part of why the book is unnerving. But for me that twist was a let down. If the tone of the first two thirds of the book was a subtle creeping dread, then the final third of the book was someone chasing you through a corn maze with a chainsaw. And that didn't work for me, nor did the twist. Sometimes turning it up to eleven unravels what you are doing rather than building on what came before. It felt like a let down. I know there are a lot of people who disagree and loved the ending, however, so it is absolutely a matter of taste.
If you want to read a book that puts you on edge and makes your skin crawl this is a good choice. If you like unreliable narrators (I don't consider that a spoiler as it seems evident pretty much immediately), horror movies, or psychological thrillers you should give this one a shot. Just be prepared to potentially be let down by the ending.