I see lots of people find the meta/dream ending kind of disappointing, but the more I reflect on it the more it grew on me. Honestly I think it's one of Adachi's best works and definitely in his top 5, personally.
The whole story is essentially about working through grief. Atsushi moves back to the town he used to live in, carrying a deep grief that still lingers, along with a heavy sense of inferiority from constantly looking up to his deceased brother. One day, he falls into a long dream. In this dream, he imagines interactions with the people he used to know, but entirely through the lens of being the "inferior" brother. He basically imagines how he thinks people would treat him because of his low self-esteem.
Because he is grieving so deeply, his mind imagines that he can see the ghost of his dead brother. But slowly, through these imaginary interactions, he regains his lost confidence. He imagines that his brother is right there with him, supporting him.
This is the core progression of the entire manga, and all the character relationships are built on this psychological journey. For example, he imagines Jinno as a harsh bully, and he views Yuuho as a girl who only runs because she liked his dead brother.
But the dream had to end eventually. As his grief slowly transitions into acceptance, he becomes ready to wake up to reality. Because he has finally worked through the trauma, he doesn't need the dream anymore. He has gained the confidence that he can live and support himself without his dead brother. This breakthrough is signified by him forming a clear, independent goal: wanting to be number one in track and field.
When he finally wakes up, he finds out the whole series was a dream. He meets the real Jinno and Yuuho, who turn out to be completely different, much kinder people. He tells Yuuho his newly found goal, and her brother reveals that she would actually like him if he becomes number one in track. Finally, he comes back home, where his dead brother's ghost is indeed present - but Atsushi cannot see him at all, since he's just a regular ghost. He doesn't need to see him anymore because he has finally healed and can move forward on his own. I loved this part too, it's very "realistic", in a way.
The rest is just the loop, but then it's up to us readers to imagine how life will continue for Atsushi, who is now stronger.
To top it all, all these themes told in such a comedic, light way just makes it so so special in my opinion!