I don't think anyone should rate this below 5, it's a well deserved drama. It will teach you a best life lesson, especially if you're going through a lot and you're in your 20's.
Spoilers ahead...
It's based on afterlife of a sucidial young adult that travells 12 body before death, and the task is given to survive; If you love fantasy and action, it's for you, the casts are insanely gorgeous. The way it made you feel chills at every episode, you can't resist. Anyone can say this is made to prevent the high suicide cases of korea. Nevertheless the meaning of life in the end indeed will make you cry.