NO SPOILERS:
This film was a breath of fresh air; It wasn't your typical horror, & what Leigh Whannell (the director) was able to create on a budget of only 7 million was outstanding. Due to the small budget Whannell had to be creative, & creative he was; every camera angle, scene composition, use of silence, & music score was masterfully utilized to create a constant sense of eeriness and dread from the moment the film starts till it's conclusion. Surprisingly & thankfully Whannell's use of CG effects was minimal, & "likely be due the budget". That's not to imply that the effects weren't good, in fact they were very good, & used well, & at the right times during the film.
Every actor was excellent & believable in their roles, but Elisabeth Moss (staring lead) was flawless. Moss stole the show, & proved she has what it takes to be an A-list actor by showing a full array of emotions throughout the film.
I wouldn't put this film solely in the horror category, but it does have a few brief & graphic moments; it's definitely more of a suspense/thriller. If you're someone who enjoys great writing, directing, acting, & an all around excellent suspense/thriller with a sprinkling of horror thrown in you won't be disappointed.