The main stand-out on this series is the acting. The chemistry between all the characters is just purely amazing. The interactions, the play of comedy and seriousness all seem so perfect. Choi Kang Hee is superb as the lead female. She did justice to the role!
The overall story is refreshingly different from other crime dramas. Or is it a crime/ romance drama? It is absolutely different from anything before.
The quirkiness of the characters is breathtaking. Well written for comedy when needed and seriousness gives it an air of a not to serious but serious Sherlock Holmes/ SH parody vibes. The comedy did well to infuse itself into the story, being well executed by the magnificent cast.
The characters are well rounded as they can be in such a short series. None are really square or boxed-in. However a problem arises here.
We have a semi kind of set-up as to the main arc and plot which does not really get developed perfectly as it should. There are more questions than solutions. Who is that? Why is she there? What is that? These are some of the questions the viewer will be asking.
Don't get me wrong, the story line progresses and there are amazingly brilliant parts every episode!
This drama could have done well as a 50 part series.
As is it is well worth the watch if you are into crime dramas with a smattering of conspiracy and some unforeseen romance that seems totally organic, and not just thrown in for good measure. 
My advise, however, is watch it closer to the airing of part two (Feb 2018) to really get the benefit of the whole story.