MASTERPIECE.
One of the best Korean dramas ever made.
The performances given by every single person are fantastic! Ji Sung and Park Jin Young couldn't have possibly done better with Kim Gaon and Kang Yo Han.
The music is perfect! You can listen to every song everyday. It is absolutely addictive. The plot has several twists to keep you on edge.
The chemistry between Kim Gaon and Kang Yo Han can BURN someone through the screen with every single glance they give each other. Their character development and the way their relationship progresses is quite unique. Their anger, hate, understanding, trust (or the lack thereof), even love towards each other gives their relationship a completeness, rarely seen anywhere else.
The lines are very well written and screenplay is also perfectly executed.