First of all I'd like to highlight how AMAZING Ji Chang-wook is as an actor! I've already watched him a couple of times on another k-dramas and he never disappoints! One of the best, indeed. :) Yoona also did a good job (not as good as him tho but that's understandable) and I think overall the acting was good. The production oh damn, I also loved it! They did a great job with those dramatic scenes, classical music is not my cup of tea but I still could feel what they were trying to convey (although they may have exaggerated sometimes, but that was exactly what they wanted, right?). About the plot & story, I'll be honest, I found myself lost as F a lot of times, but that could be just me being dumb because despite of that, there were times when things truly surprised me and got interesting and I was caught up wanting to know what would come next! I'll give it an 8.0, because despite some flaws it was a great action k-drama and I recommend it to everyone who likes action with a lot of drama mixed. :)

note: English isn't my first language so I apologize in advance just in case of eventual mistakes.