The 'twist' (spoilers)
How is it that some of you feel the film was ruined because the "twist" wasn't a total surprise? It's not like suddenly at the last minute they do a big reveal and immediately roll credits. I think you're looking at it all wrong. People seem to have this strange notion that all plot developments have to be sudden and senseless and completely out of left field. This wasn't a movie that was trying to trick you with some endgame twist that invalidates everything that previously happened. And you aren't some kind of film genius just because you thought things might be weird with this family. You're supposed to be suspicious. It's not supposed to be shocking and stunning and out of the blue that the family isn't what they seem. The fun is in trying to figure out, as the film unfolds in the early stages, just exactly what it is that they are up to (and who the guy in the car is). I thought the way the plot starts as When a Stranger Calls-style babysitter horror and quickly becomes something far more strange and ultimately demonic was the whole charm.
It's not like the movie ends after the revelation that Sam is a horned and evil thing. That would be a lame ending, sure. But it's not just some thing they tacked on at the end, it leads to the whole final act of the film (which was great). Too many horror films these days have no payoff. This one actually takes a typical premise and does something fun with it, and all you can do is whine? This is the reason why all we get in this genre is SAW sequels and clones. Any film that strays from the formula, plays with conventions, or god forbid taps into some more old school horror themes is met with hostility from self-congratulating amateur critics trying to pass themselves off as genre experts. Do us all a favor and leave horror to those of us who actually enjoy it.