If I was to be honest, I will rate this drama 3 and half. In order to objectively judge the drama, we have to agree that there are different kinds of people with various characters and some of them have been shown through this movie. Hence, judging a cast based on their character only may affect your understanding of the plot. The drama was amazing however, I feel the flashbacks were too much and I don't really appreciate the fact that the main actors life was dependent on another's (particularly an older man).
Also, the movie should have been a bit better morally. You could barely find a character you want to learn from but that's okay because human's generally have flaws. Also, I don't feel what you see about the movie online really explains it. It originally started about the OHY but transcended to Do Kyeong.
Summary is, it was a good drama despite the simple plot but it was sprinkled with a gentle, subtle twist.