I will just begin with appreciating how beautiful the story is written and how well the actors have played their role.
This is one of those dramas where not a single character seemed lacking to me or not needed.
I have watched Military Prosecutor Doberman which has Ahn Bo Hyun in the lead role and I really loved the show so I had been keen on watching more shows by him but didn't happen to pick on one before this. His role in this show is completely different from that in Military Prosecutor Doberman so I wanted to see this side of his where he acted in a mystery romcom and I am very happy I picked this show. He just made the character of Seo Ha perfect.
This is my first time watching a drama of Shin Hye Sun and after watching this I can only ask myself as to why it took me such a long time to watch her dramas. Her acting brought life to Ban Ji Eum's character. So many emotions needed to be portrayed by her and she accomplished it perfectly.
This is one of the shows I have watched where child actors are given a good amount of screen time and honestly it did not disappoint at all. All the child actors were exceptional in their role especially the roles played by Kim Shi Ah and Park So Yi. Their dedication to live up to the role is phenomenal.
It is not in all reviews that I mention the OST's of the dramas but this drama does need a mention. The OST in this show was very good and made me add it to my playlist.
Talking about the 1-4 episodes I would say that the child actors perfectly opened the story and made it clear from the get go as to how beautiful the bond was between Ban Ji Eum and Moon Seo Ha. All the details regarding the plot that should be given in the start of a drama were given in a proper pace and the story started unfolding perfectly. All characters that are needed to bind the story were introduced and explained from the beginning making the plot clear. The child actors played a major role in these episodes and you might honestly miss them in the later part of the show.
The 4-8 episodes had everything a viewer might accept from a beautiful romcom. The chemistry between Ahn Bo Hyun and Shin Hye Sun could be seen through their characters. There were many heart touching scenes and also this part of the showed the transition of feelings between characters. No major plot twists took place in this part of the show.
In the 8-12 episodes of the show there were many things that seemed to unfold. Many questions arose during this part of the show and one might sense that things got rushed in this part of the show. Uncovering the mysterious was a major focus of these episodes and the characters are caught up in taking important decisions. While the previous episodes had the building of the relation of the leads this part showed uncertainties in their past and present. The questions were all answered in the end and the story seemed to be completed well.

Now mentioning the points I disliked/liked and thought could be better are:
1. The story in the second half of the show seemed rushed.
2. 4 more episodes making this a 16-episode drama would have been an even better journey.
3. The ending seemed rushed, I felt that more time could be given to the leads in the ending scenes(I personally liked the ending that is given but just wanted more being a happy ending lover)
4. The relation of the characters with each other and other characters was shown very well.
5. The mystery element of the show could be explored a little earlier in the episodes.
6. I loved seeing Seo Ha's sincerity towards Joo Won but would have liked to see his time without her in the show the way Ban Ji Eum's character was shown struggling to get together with Seo Ha. More of his scenes during their time apart would bring more depth to their bond.

Overall, I would say it's a great watch, not many flaws that I could find. People who love romcoms especially some mystery element would really like the show.

P.S. Feel free to tag me or message me if you want spoilers as I tend to avoid giving spoilers in my reviews.