This story revealed its huge plot twist smack in the middle of the book, making it stand out from other novels in the same genre. Unfortunately, from there it just felt like a race to the end rather than keeping the momentum and anticipation up. I just wanted to find out all of the pieces and be done with it, so it lost a lot of the enjoyment after that point for me.
I did feel like the twist (trying to not reveal any spoilers!) was unlikely. If these two women are such good friends, how had none of this detail come out before? It just seems unlikely, to me.
The reader was supposed to be a bit uncertain about the main character - with her past issues and hints of darkness - but I just felt she was too thin to care about. Instead she was simply a necessary device for the plot to move around. She herself was not particularly interesting nor did I care much what happened to her.
All that being said (and admittedly it can be easier to write about the weaknesses versus the strengths!), it was an interesting story that entertained me while I read it.