What can I say? It was a great concept and was very well portrayed. Reminds me of a dark version of Frequency.
Acting was absolutely superb. Very believable performances from everyone involved and amazing work to show changes of the emotional and psychological states in the main characters.
There is one thing I noticed, however, which I have noticed in another recent film. The cliffhanger ending that is supposed to shock us but ultimately doesn't fulfill any form of surprise or dread. No, that ending doesn't work at all. Let's just say it made no sense that our villain, wouldn't be found after what happened to her. I know cliffhangers are fun, but at least make them believable.
Overall, a very well done film that, throughout has you at the edge of your seat. Just make sure to skip the credits scene so you don't end up on the floor in disappointment rather than back in your chair satisfied by a great film.