Well I recently completed this drama and will keep it short. As far as the story and plot is concerned this is an amazing drama. Both the main leads of the drama acted well. Wasn't expecting such comic role from Kim nam Gil but he was really great in playing his role as Dr. Heo Im. He was able to transform from good guy to bad and bad to guy very smoothly. He was hilarious at times and even made my heart pound with his chemistry with the female lead.
Tbh i did not like the female lead Kim ah Joon in the start. She seemed to have a cold and sly personality from her look. But as the drama proceeded I loved her acting more. Watching the drma it seems that she took her role very seriously and her relationship with Dr. Heo Im was very touching. Their chemistry will make you cry, love and adore them so much that it will make your heart flutter.
The plot of the drama first started with time travelling but the way both the characters supported each other to build and evolve into a better person was appreciable, i.e. how Dr. Heo Im character changes from a greedy to a selfless person and Yeon Kyung character evolved from a cold hearted person to someone who was empathic towards her patients.  
And the best part of he drama is that despite all the time travel b/w the Josean era and the modern Seoul it had a HAPPY ENDING!