This is by far one of my favourite crime dramas.
The first two episodes might not be catchy, but afterwards it becomes really good.
It has a great plot and an amazing characters development: Moon Hee Man, whom I hated at first, became one of my favourite characters at the end, because you get to understand why he behaves the way he does all along the story and you find out he's actually a great person.
It has some very tragic scenes but also some pretty funny ones (especially when Lee Jang Won is around) and some that make you think a lot.
There's also some romance that makes it even better (who doesn't like some romance?)
To top it off, the acting was great. Props to Choi Min Soo (Moon Hee Man) and Baek Jin Hee (Han Yeol Moo), their roles weren't easy imo but they were always on point.
The OST was also good.

So, if you're into crime dramas, you MUST watch it. If you're only into romance, it's probably not the best choice for you.