As always, Park Min Young was excellent. Na In Woo was new to me and I was impressed. He really sold his part. Everyone else was cast perfectly. The "bad guys" were fully committed, especially the best friend, and their performances were often disturbing (which was the point, I think).
The problem was the script and, therefore, the actual character development. I'm usually fine with cartoonish caricatures in fluffy / silly shows, but the good guys were nauseatingly good and the bad guys were over-the-top bad. Like, moronic, no redeeming qualities bad guys.

Given what the actors and director had to work with regarding the script, I thought it was the best it could be. Thus, 6.5.