I knew nothing of the Gintama universe when I came across this movie for the first time. I was simply browsing for something to fall asleep to during a long flight, but this actually made me fully wake up, as every frame was simply too much fun. Visual fun, meta jokes for creative types, social critique that doesn't take itself too seriously, all sorts of good stuff on many layers. Decent plot and great acting, very fitting for the genre (whatever the genre may be). Needless to say, I had to watch this movie on a bigger screen for a second time.
By itself it's already amazing, and it would have been possible to stop there. The movie did make me look up the mangaanime, too, for more context.
Review by cegg from the Internet Movie Database.