TL;DR: I would recommend this movie only if you have enjoyed both seasons of the anime and would like to watch additional stories that didn't make the cut. The movie assumes that the audience has watched the anime, so I think it would resonate less with those who go in blind. The movie is comprised of a few stories; it does not progress the main story, but instead adds more character depth through development. As this movie is also a commemoration of the 10th year anniversary, what I found amazing is how the directing, storytelling, art style, and voice acting match the original anime very closely. Ithas been a decade, yet they were able to maintain the same consistency.

Personally, I enjoyed the movie, especially the latter half. I was hit with nostalgia seeing Koro-sensei and everyone else told in a different story and perspective, There were several scenes and jokes that had me smiling giddily.

If I watch it analytically, I do not enjoy the first half of the movie as much, the narrative is a tad forced, there is one part where I find the fan service a bit excessive. Even with that, I feel the cadence of the story matches very closely with the anime.

All in all, this movie is a gift for fans of the series, it has all the charms of the series, I certainly enjoyed it.

P.S. There is a very short post-credit scene at the end, so do stay for it.