I love it.

The actors have done a truly great job in delivering their characters, who are essentially the same persons but having different personalities due to their different upbringing. I've been following YSY's works as a 2D1N fan but they were either hit or miss, but his two latest projects (Psychopath Diary and Train) have really amazed me, partly due to the intriguing plot but also more importantly due to his excellent performances. Loving his script choices and hopefully we won't have to wait for long until his next project. I also love the way SSY portrayed her two characters, they both have the same vibe but with noticeable difference at the same time. Furthermore, it's superb how the chemistry of DW A and JM A as friends vs DW B and JM B as lovers (though only shown for a bit) can really be seen just from their expressions and interactions. The rest of the casts are also no less amazing!

The twist and turn were fun, the story flow to show which world we are in was very nicely done. I shouldn't have read the comment sections every week since everyone was too good at guessing the story XD But either ways, I like how they wrap a lot of questions in the last episode. Not only with the culprits, but also about the fate and consequences (foreshadowed when DW B died), how Ms. Oh knows about the parallel worlds, and although not clearly mentioned -- how to travel between the different worlds. I think addressing it but still leaving a bit of mystery is a nice touch. I like how none of the previous details are left as plot convenience as well (as an example, Jae Hyuk's limp at the beginning serves as a nice distraction from the real culprit for the audience, but towards the end it serves as a clever storytelling way to show the consequences DW A brought to world B)! Also, the scene of the hot DW C and his complete set of boys was a very nice fanservice XDD

The happy ending does seem to be a plot convenience though. I guess the conclusion is DW was never responsible for the death of SK (death of SK A is from Ms. Oh's interference) but is fated to save her instead. But still, that doesn't mean that his disappearance or existence in the different worlds do not have any consequences and could be brushed over. Does this mean they are living their whole lives as outsiders? or does SK's statement of 'I know it's a mess rn but let's start over anw' mean they are ready to face any consequences for the sake of their love? Either way it's not really satisfying.

I also have some problem with the pacing. I really love the fast pacing in the first two episodes, but for the rest of the episodes (except the excellent finale) I feel that the progression was too slow for a 12 episodes drama. It's vexing since the story could not be explored to its full potential due to this. Another problem is with the romance between the two leads. They do have nice chemistry and the building up towards their romance is quite nicely done, but DW has never once accepted the death of SK A and never once considered SK B as a different person. I'm glad they did address this matter in the last episode, but considering the foreshadowing in JM & DW B's case, it's still very much lacking. Also, I know it's just a story but I don't really like how the supporting characters' love story (in this case DW B and JM B) and the lives of DW A's friends in world A are thrown away for the sake of the main leads >.<

All in all, although not entirely perfect, it still is a very nice drama with extraordinary casts, good editing, and with the intriguing plot built over a well-developed set rule. I really hope it was a 16 episodes drama instead :P