This movie is highly overrated. It's ranked well because only K-On fans watch this movie. Now, I am not saying the movie was not enjoyable. It was nice to see all the characters again, and the ending song was great as usual. Howver, it does not deserve to be on the first page of the rankings.
The plot is basic and seems more like a very long episode than a movie. Not that epic and I would not rewatch it anytime soon.
The animation quality is about the same as the two seasons. Nothing special.
Singing (the ending) was pretty catchy but other than that not great. They reuused a bunch of songs and none of the background music was memorable. Voice acting is the same as the previous two seasons (obviously).
...Once again, same as previous two seasons.
This is a timekiller for those moments when you can't think of anything else to watch.