This is not my kind of drama. It reminded me documentary style filming. Not making anything look beautiful, just leave it the way it is. I guess that also can be considered as a charm but it was not so for me.
This show is grey! At the start too, but at the end you realize that none of the characters is good and none of the villains is evil. Everything and everyone is just in between. OCN has some good thriller mysteries. I think this one too will have it's own audience and maybe not the mainstream one.

What I liked about this drama:

-Han Seok Gyu! Well when i heard it's a drama with him i thought i need to watch it, because I loved his acting both in Tree with Deep Roots and in Romantic Doctor. He can seem the same and at the same time so different. I like the way he talks, no wonder many try to mimic his way of talking. He played amazingly well here, and till the end I wasn't sure if he was good or bad.
-Seo Kang Joon! Well I heard about him playing in this drama first and it was a must watch for me as I love him! He played so well everywhere he was in. And being partnered up with Han Seok Kyu, well I thought this is going to be awesome. lol
-The mystery around the case at the start
-OST!! Music!! it was great really! There wasnt much of BGM during the show so it reminded me of Japanese dramas. But when there was music it was amazing! really OST of this drama is great!

What I didn't like:

-It was super slow. There are slow dramas that I like. For example My mister or On the way to the airport. But this is a thriller, I don't think this slowness suits a thriller. Maybe it's just me. At the start it was kinda confusing with two plots, one to know who is the killer and one to find a ledger and catch corrupt people. Later they merge into one. Well it indeed was predictable that they will merge into one but overall this story was complicated and simple at the same time.
-The conclusions of all the cases. Well after all the mystery around there everything turned out not so thrilling and not so full of justice or terrifying. Maybe my expectations were just too big. All the conclusions were meaningless for me.
-The greyness. I like dramas where there is no exact distinction between black and white. There are some good ones. For example the villain in Pied Piper. But here, in my opinion, they didn't pull out that greyness well. Yes, all characters were neither good or evil. they could go anyway. Each one of them could one day become the same villain. But overall I think this all could have been made in a better way and it wasn't. Everything seemed just miserable, leaving a grey taste after the drama.