What I liked:
•Son oh gong (the character), I really loved his character- strong, troublemaker, "I won't fear for anyone" attitude.
And Li seung gi portrayed that character really well.
•Bromance between Son Oh Gong and Ma Wang.

What I hated:
•Jin seon mi (the character):
She had seen ghosts her entire life so, She could have learned martial arts or other supernatural things to survive from them.
She had only dialogues in the entire drama. Always calling "Son oh gong" and demad him to protect her and also feeling guilt.
Dumb, Naive, innocent. Her character doesn't really suited this drama.
Either She shouldn't had to give guemggango or she should be acted tough without feeling guilty. It's for her survival so it's not selfish.
It can be better if her character was cold, arrogant and strong enough to protect herself.
•Soo many sub-plots:
They could have focused more on real villainess and the sub-plots could have just used to fill some remaining holes.
Half a drama focused only on killing ghosts, then after some episodes, Ah Sa Nyeo and Kang dae sung were introduced.
•The Ending:
Jin Seon Mi's death was totally a suicide. She either didn't kill the black dragon or did something useful. She rushed and get killed. Her death was useless.