I recently had watched this movie, and I must say that without question this movie is the scariest, and creepiest movie I have ever seen. In fact it was so scarry that I slept all the lights on the last two nights, and I keep a baseball bat by my night stand.
I am not going to recap the movie ad nauseum the way some of the other reviews here have already done, however I will give a high level perspective of why this movie is so good at being scarry.
1. There are no ghouls, ghosts, vampires, and demons. The premise of this movie is litteraly one group of people taking advantage of other people.
2. The costumes used by the strangers were simple, and eloquent. Exactly the kind of stuff one could pick up at any party store or costume shop. I feel as though this added a whole other level of realism to the movie.
3. Overall creepiness of the type of harrasment the strangers inflicted on the kristen, and james.
Some negatives that I observed about the movie:
1. James and Kristen making some stupid decisions, that would have any person with a brain scratching their heads...ie...splitting up when you have these psychopaths coming at you from all directions.
2. The strangers violating the laws of physics by being able to subtly and quickly sneak into a space and scrawl messages across the walls, windows of a room to further torment our protaganists, and never ever be seen or heard by James and Kristen. This just doesnt seem plausible to me.
3. The ending didnt really give me closure.
With all that, the movie was highly effective at serving its purpose which was to freak me out, and entertain me, which it did for it's entire 80 minute running time.
I highly recommend this movie, and am only giving this movie 4 stars instead of 5 for the reasons I state above.