I didn't have very high expectations upon starting this series, but ever since it was released, I've put it on my watchlist. In my opinion giving a 10 to a series would mean that both plot and acting which are the major parts of the series (for me), are really good so i guess watching this series was a 10/10 for me. It wasn't too long nor too short, just the right amount of eps.
Plot: 9/10
One of the things that has always bugged me about the whole "i would do this and that if i return to the past" scenario, is that usually no one thinks about the alternatives. There are very few stories that tend to alternatives like "what if i make another choice and it doesn't go as well as i expected it?" One thing that i absolutely loved from the very start of the series was that things weren't going as well as anticipated. All the questions that i had from the start were very well answered, and i was pretty satisfied with the major plot twists (like how the master mind behind the "reset" turned out to be someone other than lee shin, someone who was always there but was overlooked because they seemed to be the last person who would commit such heartless acts). The identity of the murderer didn't really surprise me, as the story progressed there were more and more signs hinting at them. I really liked how the characters were pretty unique and all of them had their own stories. I'd say that aside from hwang professor, kim sae rin was the most surprising character in terms of twist, and the good thing about it is that it wasn't a senseless twist. Sure, it would've been better to see more about each character here and there but it was enough for me(I really would've preferred if we had a more detailed explanation about detective park. His reasons were clear, but it would've been nicer in my opinion if they had explained his side more).
*Here's a question to mess with your mind :p : jo hyung ju didn't know about how shin gahyun was cheated on and since he returned on his own and he probably won't meet her in the same place as last time, there's a chance that gahyun would continue her love life with her fiance who was cheating on her. Do you think that she will find out about it? Even b4 the reset she had no idea about it....
Acting: 9/10
The acting was good for the most part, there were parts that could've been better though. (I'm talking about lee shin's actress) aside from this, nothing much comes to my mind
Ost: 6/10
The reason i gave this section a 6, is that throughout the whole series the instrumental osts were better in terms of setting the mood for different parts. Not that it's a bad thing, it's just that the other osts didn't really hit the spot. (But again, that's just my opinion)
Rewatch value: 5/10
Honestly there are so many series that i haven't seen yet, so generally i don't watch different series for the 2nd time, unless it's really good. For this one, the whole thing was good but not good enough to watch it again. I personally don't think that the twists would be as good to watch as the first time.

*I had a good time watching this series and I'm glad that the writer managed to not only answer our questions, but also keep it sensible 'till the end. If you like a good story with some fantasy elements then i recommend this series!