I thought the movie was pretty good. I like the way they handled it, but whoever did the costumes gets an F for unoriginality. Ever since Saw, we have seen so many movies use the creepy clown like masks to get a cheap little chill. I'm a horror fan though, so I nitpick.
As someone above stated, it did bug me how the movie threw logic out the window at multiple points. One of the strangers was shown in the house, alone with the girl yet he didn't do anything. The two main characters also had chances to kill the strangers but didn't. The final thing that pissed me off is that it was fairly evident that the strangers had access to the house and that they were in there multiple times, yet the two people being stalked always used it as a safe point when splitting up.
Overall though, if you like being scared and creeped out, go see this film. Its worth the price of admission, which is more than I can say for most of the crap that has come out this year.