This movie is for artists. At least, that's been my experience. The people who love it tend to be artists while it tends to completely lose the people who are not at all involved in the arts. I don't know if there's a correlation there, but it should give you some idea of what you'll think going in. It is VERY artistic, both in storytelling and visually. This is a very beautiful movie that keeps you baffled up until the end (then it becomes clear as crystal. Trust me). Its very inclusive and symbolic - almost everything in the movie means something and for a hyperanalytical movie-goer like me it can provide a lot of rewatch value just to investigate them.
Don't be fooled by the trailer, it's not a horror movie. Or, at least, not a conventional horror movie. One could call it a psychological horror, as it does have a lot in common with movies like Jacob's Ladder, albiet with less scare factor. Watch it to think, I warn against watching this passively.
This is hands down my favorite movie, its simply beautiful and fantastically told. But its not for everyone and some people will be confused by it. Want my advice? Watch it twice, then decide. Its a very different movie the second time around.