As a fan of similar concept, I was very excited to watch this drama. However I was greatly disappointed with the poorly done script. The writer's past work in recent years have received positive reviews, therefore I thought it was reasonable for me to expect another enjoyable drama. Unfortunately it wasn't the case. I would say this drama is a reflection of the writer's work. It contains everything we have seen in her past work. Everything was there - the funny dialogues, poetic lines and for most of us the "bromance". Sadly all of this couldn't cover up the fact that this plot was just bad.
In my opinion the story was very obvious from the beginning but throughout the episodes it was getting confusing and difficult for a viewer to understand what he/she was actually watching. Editing was also a key factor here, the story is about time travelling and parallel universe, it was important for viewer to be able to instantly recognize whether the setting/place is the past, future or present. Most of the time instead of actually watching I was just pondering upon what time/place they were currently in. Whether this was a flashback or current point of drama. The only thing that made this drama somehow bearable was Yeong's and Eun-sup's interaction.
In addition of a bad story a large amount of product placement was put into each episode. Naturally no drama is free from advertisements because it is what makes the drama actually happen. However the way the advertised products were presented was just very obnoxiously in a face that even I couldn't let this pass.
Overall this drama isn't the worst thing that happened but it isn't also the best one. I would only recommend to those who don't really care much for the detailed and overthought plot or well-written characters, but are here for the cliche. (Which isn't really a bad thing)