I just finished watching this. I cannot write my thoughts completely spoiler-free but obviously I won't reveal anything major. This is a story of a police detective with a supernatural power and a genious profiler trying to catch a serial killler that has something to do with their past. Have we seen it again? Of course. It was interesting? Absolutely.
When you came across a mystery-police thriller you wanted to be unpredictable with suspense, action (a bit) and a good case. And this drama has everything. A good case with suspense from the first episode (and it is torture when you watch it week to week) and really clever way of building and revealing the mystery. Nothing is like it seems and it can trick you. And that it is the best part. He has superpower and she is a genious but they misunderstand and lead themselves to wrong paths. As the plot was reavealing I really enjoyed the whole theme with the power abuse (from the police, prosecution, rich etc) and how they used the power their positions seems to hold to hide their mistakes.
If I find something negative is that at some points the plot was too complecated with no reason, just to be complicated. And I was a tiny bit disapointed from the final ending and the "true villain". I understand their motives and their use in the envolve of the plot and Dong Baek's character but I think was a bit random. That's the reason I give it 9.5. As for the rewatch value, I don't rewatch mystery series a second time because I know the end and I cannot enjoy them. But you can see new perspectives the second time.
Also, really nice cast with special credits to Ko Chang Seok and Yoon Ji On, with their great characters that confirm that the sidekicks are always the best.

(Sorry of any mistakes, English isn't my first language)