Firstly the cast is amazing. As Im watching this I love the story, I love how strong is the FL and also ML. There aren't many kdamas with both strong leads.
The music is better than in a dramas that are mainly based on music. I was happily surprised by this, and also it made it whole more exciting and dramatic. The music actually reminds me bit of a drama Idol:The coup It was just too dramatic with the music , but here as it's action drama, it's okay.
As I'm watching it now then after about 6 months after watching it whole i will watch it again :) it's seriously that good.