I must admit, I'm a total Eric Mun fan, and love just about everything he's done. So I may be biased. Also I do love spy stories, especially the spoofs. So this drama had two things going for it from the start.
I found it very funny from the first episode. Granted there was suspense, and not everything was light-hearted. The villains did kill some people after all. But that really didn't affect the very deft, light touch the writers brought to the story. I especially liked all the Bond references, since I'm an avid fan of those books & most of the films.
This kept the momentum going, brought in a few more thoughtful themes (particularly the examination of love and marriage, and the nature of trust), and showcased once again Lim Ju Hwan's amazing acting talents. I never found it boring, and it managed to end well, which I had really wondered about as the plot progressed.
All in all, a fun watch. I enjoy shows that don't make me think too much, and do not annoy me with hand-wringing, angst, noble idiocy, and other such annoying plot devices. I'd definitely sit through this again!