I actually had high expectations for this series because I'm grateful that SK (South Korea) is blessing us with more Sci-fi action / mystery packed series unlike before. This drama had everything that I wanted from the beginning : A solid cast (Park Shin Hye and ), A good production, An intriguing plot and OSTs that will surely land on your playlist... So what could possibly go wrong with it?? Well, before I discuss the Cons.. Let's first talk about the Pros of the series.

PROS
1. An intriguing plot : This series is not meant to be watched in a passing glance because you really need to take your time in watching this one. The mystery of the series unravels as one dives deeper in the story.
2. The pacing of the series is just right : not too fast, not too slow. We get to see the plot unravel as well as the development of the characters.
3. The acting and the choreography for the action or fighting scenes are top knotch. We get to see Park Shin Hye in action. Finally, a badass female lead and not some weak female lead or damsel in distress. (We really need more strong female leads in not just South Korean dramas but also other Asian dramas).

CONS
1. The inconsistency and 'illogicality' of the science in the show which literally may or may not disappoint you if you did not notice it. I understand that this has an intriguing plot but the execution of the science (time paradox, time loop, time travel etc. ) in the story is a bit weak. I find that the supposed sci-fi focus of the show became bland midway. Although, I can definitely say that Sisyphus had a better execution than Alice which was more of a disappointment.
2. I find the romance of the characters (Seo Hae and Han Tae Sul) to be mehhhh... I understand that SK is fond of romantic relationships in their series but I'd rather prefer if they were just friends or comrades instead. This is just an opinion but since Seo Hae and Han Tae Sul had a lot going on in the series that I actually find their relationship to be rushed or dry. Although, I want them to be a couple, it would've been better if it was on the end part of the series is where they will realize that they could've been just more than friends...
3. The ending : it is an open ending that will really leave you hanging because the main issues are still unresolved plus character and plot inconsistencies. I'm hoping for a season 2 for this because there are still important issues that needs to be clarified in the series.

In conclusion, the good balances out the bad in the series. Definitely, a slightly above average or average SK drama for me. It's still a good watch despite the discrepancies in some certain aspects but yeah, other than that. I still enjoyed watching it. Cheers to the great production and effort of all the actors/actresses, production team etc.. :)