If this film were all about surprise, it'd be a miserable failure: any viewer with half a brain knows in the first twenty minutes what the 'twist ending' is going to be. But that's not the filmmakers' focus. The concern here, I think, was with atmosphere (tense gloom broken up by an occasional splash of surprisingly ballsy gore) and character. The characters are rather generic--grumpy father, tense mother, smart-ass adolescent son, and level-headed adult daughter--but they're played in a complex and believable way. One major surprise about this film was its humor. While only one step from the sort of hackneyed observations about family life that sitcoms make, the dialogue was instead rapid-fire, quirky, and at times laugh-out-loud funny.
This will probably not be a life-changing film for anyone. But as long as you don't feel the need for a big 'gotcha!' moment at the end, it's an engaging and legitimately creepy use of a couple hours.