I really liked this show, so much so that I had to stop watching it to "fall asleep" at night because it kept me too entertained. Season 2 (no spoilers ahead) ended in such a way that I am hoping it gets renewed for a 3rd season. They slowly give you more information throughout both seasons, which kept me coming back episode after episode. They do a good job at leading you one way, then hit you with a twist.
Criticisms: I wish they would emphasize how much time is passing a bit more. I got confused a couple times. Also, they recast the character Jeong Ji-su for season 2, but didn't acknowledge it. I think maybe there could have been a way to write it into the story line or maybe that's just me. I know shows don't always address an actor change.