I watched the first 6 episodes and kept debating with myself whether to drop the drama or not. It just didn't appeal to me for some reason. Doubting my feelings I searched for its reviews at so many places and a majority of them were negative, then I read few comments on another site and most of them were about the cast so I made up my mind not to continue it.
I wanted to binge watch it and I think the special episode kind of ruined it for me from the beginning. So if anyone wants to watch it, begin from the first episode. I am not too sure about the acting because I am dropping it before my judgement is established but from what I have seen Kim Kang Woo looked the most convincing and menacing.