I loved the main couple throughout most of the show. I especially liked the more mature skinship for a change. They were lovely together through most of the drama. I'm a sucker for "we knew each other as children" storylines, if they are managed well.
Yoon Kye Sang was very good as Min Hoo.
Seo Ji Hye was lovely as Ye Sul. She is stunning and captivating to watch. The scenes when they were together and laughing were a joy to watch.
I enjoyed watching Tae In Ho as the doctor. In everything I've seen him in, he plays a complete jerk, so this was a nice change.

Problems for me:
2ML. He seemed desperate and needy and cared only about his own feelings. This wanna be triangle went on too long.
2FL. He told her repeatedly that he thought of her as a sister, she didn't care. This wanna be triangle also went on too long, and her crying and carrying on was irritating.
FL was too wishy washy. I got the feeling she couldn't decide who she liked.
Hwang Bo Ra as one of the office workers, Ye Sul's friend. I normally like this actress because of her quirkiness. In this she was just extremely irritating.
The doctor's storyline. Dating someone for a week. That storyline made no sense, and it showed up periodically to fill time.

But the absolute worst, and what made me drop my rating like a rock:
Did a different writer write the last episode?! A serious drama suddenly turned into a farce, with physical humor and by turning the main characters into caricatures! These were not the same two people that we watched fall in love and I was SO irritated!