For the non-spoiler part:
1. The time travel is straightforward and easy to follow. No advance time travel theories. No time travel gymnastics. Nothing crazy. Pretty much linear.
I strongly advise finishing the entire series first because the show itself explained the linearity of the time travel. Again, it's simple and easy to follow.
However… this is the **SPOILER** part.
They made a mistake on two things:
1. They dated the murder of Kwon Min-Ju on 1998-10-13.
2. When Han Jun Hee arrived in July 1998, her dialogue was she realised she only had 3 months to save Min-Ju.
Yet, in Episode 10/11, when Jung In-Kyu was about to jump to his death, he recalled the Christmas he, Nam Si-Heon, and Kwon Min-Ju, spent together. If Kwon Min-Ju did die on 1998-10-13, then how did he have a memory of spending Christmas together?
Kwon Min-Ju was murdered on 1999-10-13. Han Jun-Hee controlled the body of Kwon Min-Ju for almost 15 months. She spent 5 months in 1998; and spent 10 months in 1999. Not 3 months in 1998.
I'm not sure how they missed this very important detail; or why they filmed a Christmas scene and included it in Jung In Kyu's memories. If Kwon Min-Ju was indeed murdered in 1998-10-13, then the Christmas scene was not possible. If the Christmas scene was correct, then she died on 1999-10-13.
