Here's one of the cases where I would be overjoyed if I could post a review without having to give star ratings. Because I can see that the story and the characters are well written -- I just couldn't connect with any of them. So, please take the star rating for "story" and "overall" just as a placeholder.

I haven't read the manga, and haven't watched the anime adaptation, so this is solely for the movie.

First, what I liked:
As usual with Japanese productions, the use of light and colours was exceptional. It's especially noticable in Kaizaki's old workplace, where drab greys dominate -- and the light is changed according to the mood of the scene.
I also liked how background music was only used very sparingly.
The ending was also well done -- we really don't need more than the "ano" at the end. We had been told that they will retain their memories. so anything more would be redundant. I also would have liked if the film had ended after Yoake had left his reports on his desk. We don't need to be told everything, we can think for ourselves sometimes.

I don't mind so much that the six characters stay relativly bland. In a film with a runtime of two hous, there will be limitations. I do think that the other subject could have had a deeper backstory -- or the writer could have committed to only focus on the main protagonist Kaizaki and his journey of change.
Even so, none of the characters' stories managed to capture my interest -- except for Yoake, whose story I'd really like to hear. (Really, I think he is the most fascinating one, and *his* story of change, what he feels and thinks when he, the one who is supposed to be the unemotional observer, begins to like his subjects? *That* would have been interesting.) For a story about character development, it is a huge minus that I feel kind of meh about the main characters.
I can imagine that other people can relate more to Kaizaki's struggles and the story of the last year at school and their message of "Treasure the Now" than I do, so this is extremely subjective.

For me, I wouldn't have missed anything if I hadn't watched the film, but it wasn't so bad that I regret it either.