This is the best kdrama I've seen since a very long time. (probably since It's Okay to Not Be Okay)
The only serious problem I have with it is the villain's lack of personality/backstory and how they just dealt with him under 3 minutes in the finale. I could nitpick If I really wanted to, but I fail to see the point.
I always say that explaining the inner workings of time travel always ruins immersion in time travel shows, luckily in this one they have not even tried. (it's better to keep the unexplainable unexplained)

Everything else is pretty good, the cast is great, the chemistries are great. The romance is up there with the best, there are countless very fluffy and or very touching moments. Love triangle haters are safe to enjoy this as well, because the secondary love interest is weak like a straw. The ending is a banger too.

Anyway 9/10 seems fair to me. It's not perfect or anything, for example Someday or One Day is better in this genre, but I'm very happy that Korea can still deliver something of this caliber, since it's been so long..