It's nice, but a bit of a dozer. It's running almost entirely on gentle, well-observed charm, without much help from the plot. Our friends from Tamako Market are back, to tie up the romantic loose ends from the TV series. (Everyone's older. They're about to graduate from school and either go to university or work.)
The royal family of Dera, Choi and Mochi appear, but only in a mildly silly cutaway. (It's the opening. I'm not sure that was the best choice, although there's nothing wrong with the scene.)
The main film exists on two levels. One is just cute, good-natured friends hanging out together. Kanna is the film's secret weapon, thanks to Juri Nagatsuma's dozy line delivery, but Aya Suzaki's performance as Tamako herself is also unmistakable. Tamako becomes less bubbly in the second half, albeit for understandable reasons.
The other half of the film is Mochizou's love confession to Tamako. Will it happen? Will he chicken out? Will it kill their friendship? Will Tamako freak out? She's always been extremely childlike, after all, and even here her innocent interest in arse mochi and boob mochi shows how oblivious she is to thoughts like that. Anko was always more mature than her sister, despite being six years younger. Then, of course, there's Midori. The film never says explicitly that she's a lesbian in love with her best friend, but it's also very obviously placing her as the silent third member of the triangle.
You don't need to watch this film. It's nice, but low in substance. The ending's so far away from traditionally dramatic high stakes that it's as if Kyoto Animation were deliberately avoiding them, to make a point. (Mochizou could have been about to transfer away for real, but it would have felt a little corny and much more contrived than what we have here.) It's pleasant. It's very well animated. You never believe for a moment that the happy ending won't come. I enjoyed it, but I also don't expect to rewatch it, or indeed its parent series.