This is indeed a 5 star. I love it so much that it climb at the top of my favorite kdramas ever. This is unconventially perfect for me and I'll definitely watch it again.
There is no scene that'll be considered boring or just a filler. What made me love this series the most is the inclusion of the stories of the characters. Like they could've let them be perceived as 'lowly criminals' but they didn't. They included the story of their lives, how they end up in that situation, the heartwarming relationship of each of them, their growth, and the cruel reality. They've also shown how each one of us be it deprived of liberty or not experience the same but not measured unfathomable hardship in this life. Although it shouldn't be an excuse to be bad and we should use it as a motivation to strive for better instead. Because life is tough but we must be tougher lol.
This just warm my heart so much that I'll probably be happy for a week because of this.