You can be sure that rate is so high because of Kim Se Jeong - eventhough I can not say it was the best from her, in my opinion she still was amazing.
It was not bad but could be better, the idea of plot was great actually, maybe it was just too long and should have max 10 episodes (1 hour long).
Spoiler - I wished Song Jae Rim' s character actually did not became a murderer - although he is not our villain in this story. Also I feel like he had better chemistry with KSJ.