I don't want to say too much because I wanted to make this review a spoiler free one so I will keep it short.

First I think this movie is a remake (or at least they took some things from it) of a spanish movie Sleep Tight which is btw a great movie and if you are into psychological thrillers you should definitely watch it.
Okay so the plot was good and I loved Gong Hyo Jin she made me paranoid a few times, and every time she was scared she made me nervous too. She was not the strong female lead, but I personally don't mind and I know that a lot of people would probably do the same in her situation. But still If you don't feel safe in your house why would you stay there? I didn't understand that part because she had a good friend and she could have gone to her friends place for a while. On the other hand I liked that she called the police all the freaking time. Other actors did a good job as well and I liked her friend she was awesome.
This movie is a little scary, but not that much like you can get startled sometimes but I don't think this should have a horror tag it's more a mystery thriller to me. I didn't like the fact that the screenwriter did not explain why the killer did what he did and we learned almost nothing about him but maybe because the subs were bad I missed some things Idk.

All in all if you want to watch a decent movie you should watch this one and even though it was predictable to me (because I watched the spanish version) I still enjoyed it.
P.s. If you can't handle the bad subs you probably should wait a little bit longer to watch.