I'm going to keep this short (since other people have said it better); if you're looking for a thoroughly entertaining historical drama with great acting and plot, this is the one for you. Don't let the length of the drama keep you away. Yoo Ah In has done a phenomenal job in this. He perfectly portrayed Yi Bang Won's passion, his indignation, his wounded heart, his clever mind, and so much more. He has complete mastery over facial expressions. It was so satisfactory to watch. Other male leads i.e. 'flying dragons' are just as amazing.
The only thing I didn't like was Lee Sung Kyung's poor acting which ruined everything that had to do with Boon Yi for me. Lee Sung Kyung does not know how to use her eyebrows or her mouth to express her emotions and it is truly a shame. I don't know if she's improved later in her career but her acting in this drama wasn't it. I did not like Boon Yi's pairing with Bang Won as there was no chemistry there at all, but I'm still glad that romance was never the center of the drama.