Going to keep this review short. For those who watched the anime series, you already know what you're getting yourself into. All the girls make an appearance and have some sort of role in the story. The main stars, however, are main girl Mai and the mysterious Shouko. And boy, do both of these girls know just what to say and do to pull on the heartstrings of not only MC Sakuta, but also the audience. Both of the girls get fully fleshed out over the course of the movie, and satisfied fans may gain (or keep) some waifus by the end.
As for the story,
it was overall satisfying but felt a little bit emotionally manipulative at times. Can't say too much without getting into spoiler territory, but there is lots of melodrama to be experienced here. Lots of Korean drama/soap opera-level things happen to make the viewer as sympathetic to the characters as possible. Outside of a few jokes towards the beginning of the film, most of the story was very dark and heavy in tone. In fact, a grown man sitting behind me in the theater was bawling his eyes out during the last act. Things also get somewhat cliche and contrived to make sure all the character arcs have a fairly satisfying conclusion. It will be a good ending for diehard fans, while more cynical viewers may just roll their eyes at all the melodrama that happens. I'd say give it a watch and decide for yourself!
As an aside, I was somewhat disappointed that there were no new songs made for this movie. They reused the ending theme from the anime, which as much as I liked, I would have appreciated something new.
TL;DR: 7/10 - Great character moments, emotional storyline, but nitpickers may get frustrated (and/or totally turned on) by the abundant plot holes, cliches, and melodrama. If you like the original series, or also like Key anime (Clannad, Angel Beats, etc.), then this should be right up your alley. Compared to the other story arcs from the anime, this was a well above-average story arc.