Simply a work of genius. One of the scariest movies ever created. Even though I knew it was just a film, the slow and yet steady build up really got to me (and everyone else in the theater). The creators of this film should be very proud of the sound editing, the excellent use of lighting (or lack thereof), and the steady pacing. For a movie that features zero gore, and where you technically never see 'the monster' - it makes anything out of Hollywood look cheesy.
Afterwords, we were all visibly shaken for quite some time. We're all 20-something guys, who all enjoy slasher films with gusto. Even two day later, I'm still sleeping with the lights on, and the bedroom door locked. It is just that scary.