I actually enjoyed this drama but was annoyed at some serious flaws that prevented it from being extraordinary:

The plot is intriguing but the follow through is underwhelming.

Good story but the presentation was less then optimal.

R-rated because of violence - don't eat when watching this, or be prepared to ff or mute (sounds of violence disturb me more for some reason). The writer seemed to savor expending lots of time, detail and drama into disturbingly violent and cruel scenes. The writer should have focused more on character development instead.

Saving grace - it has the handsome, charismatic Yoon Kye Sang. Loved him in other dramas, in particular "Last".

Sometimes the acting lacked "feels" - Some characters (not all) didn't cry in traumatic situations that any normal human being would.
Many characters are almost one-dimensional. They feel like cutouts.

Annoying female lead. The actress is fine, it's her character that got on my nerves. She seemed to be more of a plot device making bad decisions to extend the plot.

I liked the fellow prisoners in the drama, their parts of the drama were some of my favorite. Even the warden.

Still - I kept watching.

This is a drama that could have been excellent, but with it's flaws and all it still was enjoyable. Thanks to the FF button.