It is impossible to say anything about the story because the first thing you watch is a major plot twist about the end of the anime series, and everything goes around that twist. A minor hint, though: the movie is full of Akihito and Mirai tears.
My problem with the anime series was its drama, and how it wanted to force you to feel sad or be sympathetic with its characters. That problem continues in this movie. As the movie takes itself seriously, the drama I really didn't like is everywhere. I didn't grow emotionally attached to these characters, so for me the drama presented here is just there and nothing else. And the worst part is that what made the anime so enjoyable for me, i.e. the slice-of-life, comedy moments between the characters, just got in the way of that drama, making them out of place and very uncomfortable (or in the words of Mirai, fuyukai desu).
One of the reasons why the comedy didn't work was the background music. The OST was made to make you feel sad, and basically the same sad violin song is played all along the movie: during the emotional parts, during the lighhearted parts, during the battles, during everywhere! Seriously, it is impossible to laugh at how Mirai fails to hit a tennis ball if funeral music is being played in the background. The ED song was good though. Minori Chihara has an amazing and beautiful voice, and the song is really connected to the events of the movie.
The characters also got some mild development. Even though the characterization is attached to the emotional parts of the movie, it at least answer (in a way) very good questions posed by the anime series: how Mirai got her ring, information about her mother, some mild explanation about Akihito's mother. Hiroomi continues being awesome and keeps a bit of his main character quirks. Unfortunately, Akihito and Mirai spent most of the movie crying and suffering, and we didn't get to see a lot of what made them really likeable in the series (although that is supposed to happen, given the main plot of the movie), and Mitsuki continues to be the useless, Senjougahara wannabe I've got to know.
There is little to say about the animation and art, only that it was, once again, absolutely gorgeous. Is Kyoto Animation, what can you expect?
Although I didn't hate the movie, I didn't get to enjoy it that much. It suffers from lots of the flaws of the anime series, and neither the character development, nor the amazing animation, nor Hiroomi being the best character of the series could save the movie for me.
HOWEVER, if you happen to be reeeeeeally attached to the characters, especially to Akihito and Mirai, and if you liked the drama of the anime series, then this movie would most likely be a 10/10 for you.
