1st half was awesome. Riveting and filled with so many twists that you are constantly guessing at who the serial killer is.
The middle felt a bit stupid and seemed like filler episodes showcasing the serial killer of the day.
The latter half started to get good again as it came back to the main story and we get to try to connect all the dots, but the ending was anti-climatic. They rushed to try and explain everything and it ended up becoming a bit too "easy" in wrapping everything up.

The main problem with this drama was there was just too much going on, too many people/relationships and it got very hard trying to keep everything straight. I felt like I had to take notes. Plus the timeline was really messy and confusing (I still don't understand when in the timeline, PD Hong Ju had her baby and why no one would notice that she was pregnant).

But for the most part I was engrossed at finding out who the serial killer was and how it all played out. Acting was great from the entire cast. I am a fan of Seung-gi so I mainly watched this for him. I'm not sure I would have stuck this out if he wasn't in it.
If you are a fan of thrillers or a fan of Seung-gi, then you may enjoy this drama.