Let me tell you how crazy this drama is.
Mind Boggling.
Which is to be expected of science fiction, but still I digress. Let me elaborate.

Why you should watch:
1)The Characters
So we have the main girl who is bad ass in the best way. She has the heart and she has her guns. She fights and she can be snarky. Also she makes the funniest expression. Park Shin Hye nailed this role.
Now the main guy is basically a downgrade from Tony stark, but much more funnier and relaxed. Brilliant, genius, narcissistic, but not cruel, funny, charming, and somewhat socially awkward. I never knew the actor for this one, but he absolutely killed it. I laughed with him and my cold heartstring tugged at his emotional scenes.
We also have a villain who is tragically layered and side characters with their own emotional baggage. I enjoyed the characters, though I can think of two who I absolutely hated but that was because they worked so annoyingly well against the heroes.

2)The Plot
Hmm, it is a sci fi so a lot of logic must be suspended when watching. Especially in the beginning episodes. I think initially so many things just seem borderline ridiculous and filled with plot holes. Eventually though, things actually do get explained along the way so all the ridiculous and contrived plots actually turned out to be ...real plots. You will get it towards the end. But main thing is, it is interesting enough to keep you guessing for every episode. Literally you will have no idea what's going to happen next.

3)Chemistry
This show is about so many different kinds of love, which is really its main selling point. You have the father daughter bond (which is just lovely and kickass), you have the brotherly love (which is beautifully heartbreaking) and then you have the main romance which is honestly my favorite part of the show. The chemistry is real, and it is just so well done because it is nicely layered. You actually see them start to fall for each other and you believe it when the stakes are high in the end, because you know just how much they love each other and the extent they go for each other.

4)Breathtaking scenes
The scenes are just so incredibly beautiful. The lighting, the dystopian world, I mean everything. It was very well made.

5)It has everything
It does. This drama has action, adventure, mystery, science fiction, time travel, romance, family love, tragedy, comedy. It's all my favorite genres bundled in one and though some work and others don't, it ends up being quite enjoyable.

What the potential drawbacks are:
1) Can be draggy
Some scenes are just unnecessarily long. Or maybe I'm just not an action person so I felt the show dragged in some long action scenes though it was nicely done. There are also moments where the main female lead takes her time wandering around some places which could have been shortened.

2)CGI is....not the best
Honestly some scenes (especially the beginning) are just so ...... bad. Like laughable bad. But if you can ignore it them and allow yourself to suspend some logic, you will find yourself enjoying this a lot.

3)The Science-y Stuff
Okay I got lost somewhere in the end. Trying to figure out what exactly, how exactly and just....what? Though there is some kind of rule they use that makes sense, the drama ends up piling different possibilities together that it just end up confusing . Or maybe I missed it because I tend to zone out anything too science-y related.

4)The Directing
The plot is amazing. The cast is amazing. This show could have been phenomenal if it had better directing and execution. That's my only gripe. So much potentially which kind of withered away because of some not so great directing choices.

Overall I highly recommend this one. Some episodes where better than others for sure, but I enjoyed the whole experience and its unpredictability. Just don't watch it for the logic. Watch it for the entertainment and you will get it and more.