Just saw this today and fell in love with the story. Even now as I am writing this review, I still don't know which parts were real and which ones were fictions of the main girl's imagination. I love it. It's smartly written. The last sentence in the movie really bothered my mind.
The only thing we found unnecessary was the obvious (and sometimes false) jump-scares. The story itself is good enough to bring out the horror of the film.