My judgement of this show is based purely on the Korean version, as I did not watch the original. (I might do to see if its any better)

Disclaimer: This review contains spoilers
(I totally forgot the names of the characters)

In the beginning (first few episodes) I kind of enjoyed the show. Usually I am a fan of fluffy slow burn romance. However this was just too immature and slow for me. There are just many things that bothered me with this show.
First of all the main characters. Both of them are very childish and do not develop much throughout the story.
I really dislike the tsundere nature of the male lead. Sometimes this personality trait can go well but in his case it was just very off putting. Instead of teasing the female lead he just behaved very rude as if he did not like her at all. The after end credits made me sympathize with him a bit more but overall he was just very unlikeable.
The same goes for the female lead. I found her quite insufferable because of her immaturity. It seems as if she has not aged at all even after the countless time skips.
Which brings me to my next point: the time skips! Eventually they just got on my nerves. They just made the story unnecessarily confusing and were used way too often. This made me really dislike the pacing.
Another aspect I did not like were the side characters.
Firstly I hate love triangles so I did not appreciate the fact that there were two male leads. If I am being honest I liked the second male lead a lot better. He was always there for the female lead whenever she needed him and overall just treated her a lot better. Even though she made her intentions clear (liking first male lead) he did not give up and treated her the same. (I was really rooting for him)
Secondly the side characters just had horrible development even worse than the main characters. The relationship between female leads friends felt so forced. At times it felt like the friend did not even like her boyfriend. The fact that they were still together after college and even married was so surprising. While they were dating in highschool she tried to have an inappropriate teacher-student with the school nurse and that whole situation was brushed off so fast. I truly hated their relationship and think it is one of the worst ones I have ever seen in a K-Drama.
Overall the show was just not for me. I simply dislike shows where adult characters have one small kiss at the end of the show and act all shy and embarrassed when the accidentally touch each other. I really had to force myself to keep watching it in hopes that it would get better. (It did not)