I don't know where to start, because this is probably the best drama I have watched in a long while. So to start with the nutshell: If you're into crime, if you like complex and not entirely satisfying (everyone is happy, justice is served, amen) storylines, and great acting, then this is your drama.

Plot:
The tone and stories are, in my opinion, darker than season 1, but I prefered that. There's not a single case with easy answers, and in at least one of them, the final solution feels realistically unfair. Characters are multifaceted, and often victims and perpetrators are not that easily categorized. So, great plot!

Shen Yi's art is used in more varied ways in this series, and some of the statements about art... They took some liberties there, and, call me a philestine, but I don't get the map he's trying to draw. But still, the ideas are really ingenious and set the drama apart from other detective dramas.

Acting:
The two leads, Tan Jianci and Jin Shijia are really the showstoppers of this drama. There's some brilliant acting going on! They had me on the edge of my seat, crying and laughing and awwing. The dynamics between the two leads is probably the best reason to watch this drama apart from the cases themselves. If I could I would shower them in awards!
But it's not just he main leads' performance. In fact, the acting in this drama is on point even down to minor characters and guest roles. There are some really remarkable performances of suspects and victims here.

Re-watch Value:
In my opinion, both seasons are really worth re-watching, because they contain more depth the second time around.

In a nutshell: Brilliant acting - realistic and consistent characters - great script - interesting, challenging cases - great title song - some real cliffhangers - and addiction factor 10/10.