This was one of the first KDrama's I have watched while it aired, and let me just say that every episode kept me on the edge of my seat, and I was SO disappointed having to wait each week for the next two episodes!
Yoon Shi Yoon is quickly becoming a favorite actor of mine; he has quite the range and this really showed when playing the two different versions of the Seo Do Won. He had a similar acting experience in "Your Honor," where he played twin brothers, but this show and these characters were much better in my opinion.
The plot is complex but well paced and understandable, and much better done than "The King: Eternal Monarch" which had a similar premise but was executed in a much more confusing manner. This is also one of the first KDrama's that I have watched that hints at/needs a second season - very badly!
Overall this was a very enjoyable watch with great acting, well developed characters, and an addictive plot.