I watched this movie late last night (alone) and it really freaked me out. Although, in my opinion, the movie needs to be reclassified as a horror rather than a thriller. The storyline is very unique and extremely realistic. I am not a horror buff but this one grabbed my attention. Although the story develops slowly, the movie never lost my attention. The actors and the cinematographer deed a very good job. The black and white flashback sequences were very disturbing. But as some other viewers already commented, there were some plot holes/things I didn't get. But I am guessing that those questions were done on purpose and the writer wanted to keep a certain degree of ambiguity. Upon reviewing the movie the second time, a lot of questions seem to have been answered very subtly.
Bottom line was, I really enjoyed it, it was creepy and would highly recommend this one to my friends.