Let's start with my overall view:
To my eyes, as a mere kdrama fan, this drama was made with serious budget. The visual effects (CGI) were quite seamless and beautiful. I liked how smooth the switching takes between human-robot Nam Sin were, and I loved the action scenes! All-in-all, put this in your watch list if you haven't done so ^_^
Story:
Now, I'm a fan of sci-fi. I have seen the classic "AI" and "IRobot" and I loved them, so I would say that the story itself is not entirely original. However, I do find the screenwriting interesting - thumbs up for the screen writer - though I find many of the "decision" made by this cyborg can, in fact, only be made by human ^_^
Cast:
I love them! All main casts' acting was very good, I enjoyed seeing them portraying the characters. Kudos to Soe Kang Joon, as he successfully played two characters of opposite traits. At certain points, he managed to made me feel like punchin' the human Sin in the face *lol* That's how successful he was in playing the characters.
Music:
OSTs are beautiful and the scores fit the scenes well enough.
Rewatch value:
Well.. I never rewatch drama series.. ever.. so no comment on this one. hahaa..