I love Ruth Ware, she writes the perfect mysterious thrillers and this was one of them. I agree with the biggest critique I've seen, this is too long. There were a lot of chapters in the middle that could have been cut or condensed. I also would have loved a more thrilling end. 
However, I never saw the twist coming until it was shoved in my face. This is one of the best "dark academia" mysteries I've read.