This drama started off strong having an intriguing plot line. However the more I watched, the more it just went downhill.
There were more cons for me than there were pros. For instance although the plot itself was a good baseline, the background concept and "rules" of time travelling set in this drama was hard to follow, unless you really concentrate and think about each scenes.
The drama's CGI and background sets were pretty good and I was quite impressed with them, as the post war environment for instance really looked like a dystopian era. However, the same could not be said for some of the stunt scenes played by the actors, like the bridge jump scene or the zip line scene. They pretty much looked very fake and you can see how unnatural their falls were.

Further criticism of this drama is that although the drama started strong, leaving viewers hooked on the plot and what would happen next, as you watched more and further into the drama, the less plot driven it became and focused more on the ML and FL's romance. The focus of the drama started with the ML's brother and his investigation on Sigma but this plot point eventually got lost and not much further development pursued. I would have much preferred to explore the plot point of the ML and his brother's relationship and brotherhood and their struggle against the villains than the romance tag. The romance in all honesty was not needed.
Speaking of, this is especially so when the ML and FL don't have that chemistry that lovers do. Watching their romance scenes didn't give me that butterfly in your stomach feeling but left me with the impression that the pairing was forced and they were then quite awkward.
The Second ML (Sun) was also a character that didn't give me much impression either and didn't need to be in the plot point. Although towards the end he played a relevant part of the ML's overall plan, but it was something a different character could have done. For instance the police guy that was seeking revenge against the FL due to his mother. His character could have had so much potential if only the drama gave his character more depth.
Throughout the drama, the romance scenes overall was very awkward for me, even the kiss scenes. Furthermore, usually in dramas that have love triangles, I normally enjoy it when the 2 ML's try to win over the FL's heart. But in this drama, although the love triangle was there, it again feels forced and both ML's are quite childish that it isn't very enjoyable to watch them try to please and win the FL. It instead looks forced and awkward.
Both the FL and ML also keep making emotionally driven decisions that it ends up being repetitive and frankly quite annoying and end up feeling frustrated at the scenes.

In all honesty Kim Byung Chul ( who played the villain) and Lee Woo Won (who played the child version of the villain) carried the whole drama and were the reason I even finished all 16 episodes. They both portrayed their characters really well especially with their expressions to bring out the mental issue and psychologically disturbed setting of the character they played, that it made watching the villain the only thing you are looking forward to each episode.