Hmmm, how to review this without spoiling any of the plot twists...
Okay, overall, definitely a thumbs up. Very suspenseful and spooky at times, borrowing a lot from the Blair Witch Project and from Hitchcock in that a lot of the scariest stuff happens off-screen, so it's left to your imagination to determine exactly what's happening. Relies on good acting rather than gore or other hokey special effects. I found the ending to be less than satisfying, but in this type of movie that's almost always true. You can't really wrap things up in a neat little ball without it seeming trite and canned, but neither can you leave things hanging with a lot of unanswered questions. That's what I always found so annoying about Michael Crichton's books too, they build such an interesting plot that any ending would seem a bit anti-climatic. That's definitely the case here too. For the entire movie you'll be paying very close attention trying to guess what's going to happen next and it's time well spent. A step above most other suspense movies even of it lacks the great ending of The Sixth Sense. Go see it, an excellent film.