I judge how good a drama is based on the emotional payoff at the end of the series.  If not for the terrible ending, I would have given this series a higher rating.

The drama spent a lot of time exploring humanity and choices, and how our choices can affect who we become in the future.  Unfortunately, for a drama involving criminals, police, and profilers, there is very little about justice and the consequences of those choices.   Some villains from the beginning of the drama were never mentioned again.  

The romance in this story was unnecessary and awkwardly included in the drama.  A sign of poor writing.

The acting across the cast was excellent.  From the child actors to the main male leads, everyone did a great job and conveyed complex emotions properly.   Watch this drama for the eye candy, and enjoy their acting while you are at it.  Seo In Guk did a great job, as did Park Bo Gum.  Unfortunately, I really couldn't understand either of the characters nor could I understand what the writers wanted us to feel from watching.  I don't think anyone could be like Seo In Guk's character and be as levelheaded as he was, even when facing murderers.  Were we supposed to feel pity for Park Bo Gum?  I agree with many reviews that the true chemistry is between Seo In Guk and Park Bo Gum.  When both of them were on screen the screen sizzled.

All in all, it felt like the drama was trying to make us feel more and think there is some hidden message.  But it was not well written enough to properly convey whatever those messages were meant to be.  At the end, you got a serviceable crime drama with an element of melo (hidden identities and such).  The good acting made the drama worth watching, but don't expect this to be a masterpiece.