Gintama is one-of-a-kind, it's hilarious but can be serious and sentimental when it needs to, and towards the final movie that wraps up the entire storyline, it manages to be everything it wants to be. It may not be the One Piece or Naruto that the masses love, but if you watch it and stick with it, it is one of the funniest anime you could ever watch.
Of course, this movie is really the finale of the series, so going in blind is not a good idea, it's meant to serve the final chapters from the manga.
Start with Gintama and embrace its silliness and you won't regret it.