From the very beginning, the initial scenes felt shallow and unreal, particularly the attempts at portraying a romantic connection. Even when the story moved forward in time, that feeling didn’t really change. The romantic elements were excessive, with drawn-out, hazy sequences that only served to make the already long show feel even longer. A predictable, uninteresting antagonist was present just to create some conflict. I felt Bae Suzy wasn't quite right for her character; she lacked a certain strength and intensity needed for the part. No matter how she tried, she didn’t seem capable of appearing threatening or powerful. Sung Joon is usually a more skilled performer, so his performance here was disappointing.

Source on TMDB