This was the first Korean drama that I've ever watched so I went into this blind. Actually, I found it by accident while searching for another series called Dr. Stranger, a friend recommended it to me a long time ago (I watched this instead). I definitely want to explore more K-dramas in the future!
I love all the characters, I don't think there was a single one that I didn't like (although the North Korean villains were probably the weakest part, even then I still found them quite threatening). The character relationships and dialogue were probably my favourite part, although I also really liked the costuming and cinematography. At some points, I forgot they were actors.
As other reviews have already stated the plot sagged a little, but it was still very suspenseful (a little bit like reading the Harry Potter books for the first time) and I loved all the characters even more. Even though the premise seems ridiculously contrived, it was executed really well!
I can't say how well North Korea was represented, the show didn't seem to be glorifying it at all. At any rate, it sounds like a terrifying place to live, though it wouldn't surprise me if it were worse in real life.
This may be because of my unfamiliarity with K-dramas (or because of rumours I've heard of North Korea), but I couldn't tell at all how it would end if it would be happy or sad or bittersweet. (I won't say here though) The ending was good but it dragged out a little. This was my first time watching a series where I didn't want to keep watching because I was afraid of what would happen next.
Despite some minor flaws, I thoroughly enjoyed the series, though I don't want a season 2 because the story seems to have ended. 5/5 stars.