This movie is underrated, my perception is that if you go into this movie expecting to be scared at all, it will disappoint. It is not Signs, it is not Sixth Sense or Unbreakable. I initially watched this movie as part of a chapter on dystopian literature in English class.
This movie is suspenseful in the sense that it takes a long time to connect what is actually happening, and then you realize that you were wrong. It is not suspenseful in the sense that you will be scared waiting for the next step, if anything you will be confused in a good way. If you go into this movie having no expectations for genre, and you are willing to lend imagination, the concept is very nuanced.
There IS a massive twist in this, it's just not a terrifying one. The emotional and romantic aspects are moving and deeply thought-provoking. I honestly wish that Shyamalan did more outside of traditional thrillers because of this film.