I really wanted to like this more. It started great, with an interesting storyline, some humour, and inklings of good chemistry between the leads.
As the series progressed all we were left with was the interesting criminal/legal storyline - although even that one was poorly executed and lame. Apparently there are no court psychologists, lie detectors or any other kind of tool in the Korean legal system. Also, the serial killer investigation is led by the prosecutor (and some random lawyers) and without police involvement? The simplifications done to these processes is really what brings in the lameness.
If the chemistry between the actors was better built, perhaps it would have been a more intriguing watch. The female lead ends up baby-talking through most of the series (ok, her voice does naturally tend to that end of the scale but still..) while the male lead pursues some kind of constant and monotonous nonchalance throughout 20 episodes. At the crux of it, the lame almost emotionless statements of "ok, so let's be together" and "are you proposing!?" really underscore how much of a missed opportunity this show is. Anyway, it is watchable due to the criminal and legal storyline despite the shortcomings.