It's a slow burn, building a manic obsession up to a shocking climax.
Great story and solid performances from all, building the tension throughout.
Rachel Warren IMO steals the show as Bex, transforming from normality to utter psychosis as the story unfolds. Wow, just look at her as she completely flips: have you heard a scream of desperation and anger quite like that?
Overall, pretty disturbing (that's what good horror is I guess) and left me a bit troubled which is, after all, the point of the movie.