Honestly, I'd put off watching this. Not because I thought I wasn't going to like it, because I knew I would, which is why I left it for a year on my to watch list. I wanted a time to savor it. Stupid me though decided to binge it during high stress this week though.
So first, take my advice and savor this if you haven't watched it yet, because it deserves full appreciation.

Now onto the spoiler review.

Cast:
Well done. Honestly, I'd only recently started to get into Ryeoun (Adult Trainee ep 3-5 is amazing), and I'd been a fan of Choi Hyun Wook since 2521. I knew they were going to give me stellar performances. And they delivered. They played the father-son/bromance so well. They had such beautiful vulnerable moments that left me gasping. They played amazing as leads. But I think the star of this show, for me, was Shin Eun Soo. She conveyed so much with her expressions. I was always waiting for more scenes with her. As a cast, they all did phenomenally well. I loved the band, who gave so much camaraderie and youthfulness to the show and the grandmother who supported them. They all complimented each other very well. The chemistry was wonderful.

Story:
The show is about living and shining in your youth, and how you overcome your obstacles. But it's also about having a fulfilling adulthood - your life isn't over after highschool, and I think the end really emphasizes that. HEG's parents still existed in the present, and they still shined brightly.
The story starts by showing us HEG's struggle throughout his childhood as he was his parents interpreter and their only link to the outside. It's a heavy burden to put on a child's shoulders, and you can tell that it isn't one intentionally done. Largely it's because of the lack of support, the feeling of being ostracized when you are disabled, and the lack of resources when you are poor (which, unless you are a wealthy heir, is very hard to overcome when you are disabled). It is clear that HEG feels burdened and doesn't want to - he loves his family. And it is also clear that he is unable to communicate with them despite being their only link to communicate. We know as an audience that if he expressed himself, his family would be understanding and supportive, but to him, it's almost insurmountable to actually be selfish.
HEG's chance to go back in time allows him to connect with his parents for the first time, and really speak to them and learn about them. He gets the opportunity to express himself and be accepted, and to right the wrongs in his parents past, saving them and himself.
I love that the story makes HYC's accident a fixed point. I think that it's important to show that it wasn't something in his destiny that needed fixing.

Music:
Very 90s with a hint of modern. I was a big fan. And you can see that they had great stage presence during their shows. I loved HEG busking on the street in the first episode and I think their charisma was very well done to portray them as musicians.

Cinematography
Very bright and youthful, even in the moments that were sad. There was always the draw to the light.

Rewatch value:
First I'm going to watch all the fan edits, then I'll rewatch.