Short answer:
Overall the film is a good thriller, one-sitting and action movie where you don't have/need to think about much of it. Let it flow and enjoy the film even though some part of the movie doesn't make sense. It's not a bad movie, but an average. Watchable with suspense and thrill, but not memorable enough to be favourite.
Long answer:
Acting, cinematography and CGI were excellent (I'm not going to explain it due to general idea and comments that are similar to other reviewers that you going to find here. Rather, you should watch it).
The first 75% of the movie was amazing, however, the ending is a bit problematic (and some other part of the movie that doesn't make sense). Here's the list (no spoiler ahead, maybe a bit):
1) Ending. It felt rushed when we got to the conclusion of the movie. I was expecting something that the ending going to give a paramount impact, not a dreadful one. Why the "magic" happen? I this to teach her a lesson?
2) Plot twist. The plot twist was a bit predictable, not convincing enough. The movie's plot didn't get "marinade" enough to send into a new scene and information.
3) Time travel, paradox or time paradox. It was one of the biggest themes of the movie. It is a tricky theme to play with, but if you nailed it, it's awesome. When changing something in the past, part of the memories are changed and the later person in the future already knew the answers of it. It felt that the movie did not portray this accurately and purposely for the mental side of the character but rather focus on the physical effects of the paradox.
4)Teleportation issue. Here where it gets tricky. (Spoiler alert). The main character, Seo-Yeon, can't drive, living in a house far from the city/town. How does she get to places quickly? The transitions felt rough here, the movie did not give a sense of distance on, and also, time.
5) Supporting characters. We didn't get to learn other characters and how are they significant to the plot. We didn't know about Young-Sook step-mother and how she became her mum, or who is Sung-Ho?
6) Character development (in relation to the ending of the movie, and yes, spoiler). Seo-Yeon development did not reach my expectation. Like the ending here she met again with her mum, giving me a sense that the story might be a love-hate relationship with the mother? Because by quarter-end of the movie, we were surprised when Young-Sook said that it was Seo-Yeon fault for turning on again the gas after her mum closes it, and blame it on her mum. Finding her mum alive, and appreciating her just drags me away from the other part of the movie, which is Young-Sook and the time paradox theme. What exactly is this movie trying to say? Appreciate the parents? Appreciate people that you loved? (But who is Sung-ho?)