Admittedly I've only watched the first 6 episodes, but I'm really enjoying the story up to now. I can see the similarities to Death Note as another reviewer mentioned, but I was reminded of the old movie 'Curse of the Demon' based on the story Casting of the Runes by M.R. James. Our decent heroes stumble upon a strange world of witchcraft where they need to get up to speed on the rules of the game pretty quickly before some truly nasty villains catch up with them and do them harm.
I'm relatively new to Korean dramas, but if you enjoy tales well told (and who doesn't) then give them a try. The stories tend to be more fantastic and melodramatic than their western counterparts, but I enjoy thriller/mystery/fantasy shows so I'm all in. Acting is as good if not better than in western shows and the writing in the shows I've watched so far (Dr. Prisoner and Signal) is well above the usual standard I've grown accustomed to. Perhaps they have trouble spotting the landing and getting the ending of the final episode of their shows to be as satisfying as I'd like them to be, but I'm so busy enjoying the journey that if the ending doesn't quite hit the bullseye then it's there or thereabouts and I'm entertained enough to search for my next Korean show to sample.