It's really great drama, but not greatest. in the beginning episodes you will think you found one of hundreds level drama, though second half isn't that level. in first part there is real tension, twists and turns, unexpected turns, every element of really high quality greatest drama, but in second half kind of nothing new, same and same "searching saving her" and "madly in love ML". in the beginning I thought it even could compare (not same level but could be close) to Cruel City, but at some point it becomes repetitive, by episode 10(+/-) I already knew there wont be anything new and big anymore, he will search her and love her till the end.
I'm not saying ML should give up on her, but after first few episodes you have expectations that this isn't usual action or usual romance, you expect more, because first half is really great and different, and you think second part will show also great plot and unexpected events, turns of story and character, like in "Cruel City" or "Kairos". just "saving girl he loves" becomes repetitive and for me disappointing, I was waiting something like it was in Kairos, when at the half way ML realizes she wasn't who he thought, or dynamic changes of situation like in "Cruel City"... but here actual "status" kind of stays same for 16 episodes: lost girl, searching/fighting for her ML, madly in love ML and modern Romeo/Juliet vibe.
Drama has one of greatest potential in first half or at list first episodes but then it's just another good action/saving FL/revenge drama.

I'm not saying not to watch it, on opposite it's really good and way better drama then 70-80% of kdramas, I'm just saying it could be better then 90-95% of kdramas.

p.s. I can forgive rom-cos for cliches, but high level dramas, when will you stop showing ML's like almighty supermans and bad guys with "evil laugh"???