This drama started strong for me, but ended in such a unnecessary place. I'm not sure I understand what the show was trying to say or accomplish.
Personally, I think they should've cut the whole 2023 storyline and kept the focus on the kids. I didn't hate the 2023 storyline, but it was distracting. Also I strongly feel that they ruined Go Yoo's character in the future because they wanted to keep the audience guessing on which boy it was.
I also think they should've cut Han So Yeon's character. She only added conflict to the story, which is usually a good thing, but in this case it was more frustrating than anything else because I loved the relationship between Go Yoo and Joon Hee and she was only getting in the way of them. In fact, their friendship and brotherhood was the best thing about this drama and truly the driving force of the whole thing. Which is why in the end, when Joon Hee leaves (after experiencing more headaches that are never explained) it makes no sense! Why would he stay gone for years and years? He wouldn't.
Lastly, since everyone is talking about Sehun, I want to throw in my two cents: I think he did a really good job. I went into this having seen a few of his older dramas and not being very impressed, so I was pleasantly surprised. This was a great role for him. He had great chemistry with all of his co-stars and was, in my opinion, the heart of the drama. I would love to see him take on more roles like this one in the future.
In the end, I'm disappointed with this drama, but it was short and had enough heart to keep me watching until the end.