Well, lets leave behind the fact that the drama was tedious in some parts because they want 16 episodes 1 hour each, just for 1 serial killer.
I didn't like the end. First, they caught the killer in the present so he had to catch him again in the past(at least he knew the way). Secondly, I was happy he made it to go back to his wife and child but what about the others? The rookie from the past didn't become chief? Kim Seon Jae did not become detective and did not meet Shin Jae Yi? I mean everything changed and he have no idea how. At least they could have shown everyone 30 years later and what they were doing. This would be the perfect end.
Anyway, I really enjoyed this drama but if the end of a movie or drama doesn't satisfy me....I mean that's what the whole thing was about. Dont screw it in the end!
The reason i put 8 as rating and not higher is because i don't think it deserves 8.7(current rating). If you wanna watch something simiral with fantasy then watch "He is phycometric"(maybe this will make you cry a little bit more but defenitely worth watching much more)