I enjoyed this series quite a lot. I don't usually like k-dramas because the romance is extremely slow-burn and the drama is too over the top, like in telenovelas. What's different with this one is that the focus is not on the romance. It has a captivating story that will keep you wanting to see it through to the end.
Sadly, some characters are very annoying and the events get a little too ridiculous in some of the last episodes, which almost made me lose interest... but, trust me, it's worth watching till the end just because the main conflict's resolution is very satisfying. It will not disappoint.
As for the main cast, Park Sae Ro Yi comes off as strong and inspiring whenever he's in an important scene; the rest of the time, he could be replaced for a wooden plank and we wouldn't notice. Oh Soo Ah's motivations and feelings are confusing and Yi Seo can be really annoying at times. It looked like they tried to make us feel some empathy for Geun Won and I thought he would receive some character depth, but they completely ruin it in the end. All of my praise goes to Yoo Jae-Myung who made a great job as Jang Dae Hee, the main villain; ruthless and dignified but with clear motivations and foreseeable weaknesses. Without him, the series would have probably been a flop.
I dropped a couple points because of the problems mentioned above, but the series has just enough jaw-dropping moments to make up for it and make it watchable. Plus, the OST was the best part of it. Ha Hyun Woo's voice is amazing, and his theme "Diamond/Stone Block" along with Gaho's "Start Over" are definitely going into my playlist. Worth a watch even if you don't like K-drama.