It's like watching a puzzle slowly put together with each new piece/episode providing new inisight to the bigger picture. Everything is brilliantly connected and every character matters and is involved in some way.
The show is not just about finding out who the killer is but most importantly about the people involved and how deeply they were affected by it.
I don't see an issue with the pacing, it's what you would expect from a psychological thriller, no time was wasted. It slowly builds and forms the story with each episode and once you get to the end you see just how perfectly thought and planned out it was.

The cast I can't praise enough, all of them were phenomenal and had me so immersed that at times I forgot to read the subtitles, especially Shin Hakyun who blew me away every episode.
The ost seemed a little odd at first but I quickly fell in love with it and thought it perfectly fit the show.

No words are enough to describe how much I love this show and I'm not really good at reviews so if I could only use one word to describe it it would be masterpiece, it's by far the best k-drama I have seen.