Keep in mind again, this is a review with the original source in mind, so there will be a lot of comparison. I'll try to be as objective as possible.

I'll start with this. I would've preferred watching this without reading the webtoon first, because I couldn't watch it without comparing them. Since I want this review to focus about the story, I'll point out the other stuffs first.

Cast:
The casts did a great job at their respective roles. Shin Hye-Sun portrayed Ban Ji-Eum well, especially her quirkiness, her intelligence and charisma. Ban Ji-Eum is a unique character and she does well to make me believe that I see a live-action of Ji-Eum with her own touch as well. On the other hand, Moon Seo-Ha is a really delicate character. His current character was shaped by his "never-ending" trauma he got from his past and I would say that I'm really impressed with Bo-Hyun's portrayal of Seo-Ha, especially with the early skepticism that I had. Other casts did well as well. Their chemistry were also great.

But honestly, if I can point out who impressed me the most, it would be Kim Si-A as Ju-Won (Ji-Eum 18th version). Her mannerism, her glares hooked me as she looked exactly what I imagined Ju-Won would be. Her acting literally gave me goosebumps.

Music:
At earlier episodes, I would say that I hate the background music choices they made, especially with loud-comedic music. The music made it seemed like that Ji-Eum's actions (especially with Seo-Ha) as jokes and we should never take it seriously, but she did it with purposes and actually was serious even though she did those light-heartedly. The music didn't capture that essence. The later stages of this drama, the background sound choices became much better. Other than that, the soundtracks themselves are great and will definitely listen again.

Story:
Okay, this is the part that I want to talk the most. The earlier episodes presented the past and the problem well. The characters were introduced tremendously, especially Ban Ji-Eum's character. Also, this adaptation incorporated Ji-Eum's past lives really well to her current life, better than the original source. Seo-Ha's trauma was also potrayed well. From the beginning, it was clear to me that there would be differences between the adaptation and the source. In the earlier episodes, the changes weren't apparent and they only altered the ordering of the events, which I thought was fine for the most part. But as the story went on, the gap became much larger, and it seemed later they made a new larger. They introduced new characters, they altered the story, and removing some of the better parts of the original. As a fan of the original story, I was honestly disappointed.

(Warning: The following paragraphs will be spoiler heavy, so read it with your own risk. You could read the spoiler free conclusion after the *...*)

*There were some changes that were good. Like when Ji-Eum admitted to Cho-Won that she was indeed her sister reincarnation. It was one of my favorite moments from this series. Also, the fact that Cho-Won is a florist instead of working in entertainment industry was a nice direction. Other than that, as I have pointed out above, they incorporated her past lives better in this adaptation.

But for the most part, I think the changes made the story worse than the original sources. In the earlier episodes, since they altered the ordering or flat out removed some of the events, the flow of the story felt so choppy. In this version, it felt like Seo-Ha overcame his trauma so fast, and it just went away for the most part when meeting Ji-Eum, even though it lingered him since his accident. It's pretty disappointing how "easy" Ji-Eum made Seo-Ha fell for him.

Other than that, it is pretty disappointing that we didn't get a lot of moments with the 4 main characters, especially interaction between Do-Yun and Ji-Eum. Also, I'm not fond of the direction they went about mushing two different events at the same time. In the later episodes, they tried to tell two different important events happening at the same time, even though it didn't have to be like that. I understand why they made it like that, since the series is just 12 episodes (should've went for 16 episodes to begin with), but the execution wasn't satisfactory.

A noticeable difference between the two versions is that the original source focus firstly on how Seo-Ha forgiving himself and his past and then gave sprinkle of Ji-Eum's past lives a bit later, but in this adaptation, they put so much stake in Ji-Eum's life, especially regarding her past lives, and made this Ji-Eum much more vulnerable. Seo-Ha was much more confident in contrast. I don't think it made story better or worse, so I don't really mind.

What I do mind though is the ending. Spoiler alert: I hate how they made Ji-Eum forgot her past lives and the people surrounding her. It didn't make so much to begin with, because altering Ji-Eum's memory means that her life changed, and it didn't make sense that to me at all how she would not know or remember that she worked in the hotel before, knowing that she had a friend in MI who weren't not related with her past lives. Wouldn't this friend be surprised that Ji-Eum came back working there?
Also, what about her childhood? How would they fill the gap that big in her memories? Especially since most of her childhood was spent with Ae-Gyeong. Unimaginable. She spent so much time with people from her past lives, so it's unbelievable that she could be in a livable state with that much gap in her memory. It made the ending felt rushed, especially without more explanation about this. Altering her memory without altering everyone else seemed weird. Again, the approach for the ending given in the webtoon made so much more sense and also much sweeter.*

Overall, as an adaptation, I would honestly have given the story a 7 or even a 6.5. If I hadn't read the original, I think I would've given this a 8.5. Even without the original knowledge, I would still be able to see the flaws such as the flow of the story itself and also some of the weird contradiction and gap in the series. But I would've definitely enjoyed this more without the former knowledge. So, as a conclusion, as a fan of the webtoon, this is not a good adaptation, but as a fan of Kdrama, this is a good series.