Back to the topic of the thread ... yes Margaret, their operettas really just work, especially the really famous ones (Mikado, Pirates, Pinafore, Patience) and they'll be played the world over for a long time to come.
I'm a bit of a devotee of the final two, however, they suffer from a misunderstanding audience that just didn't want them to develop their style. The music was far more advanced and the lyrics were much more obscure.
I think Sullivan's need to move on and write an opera was necessary, even though it's never staged anymore. Queen Victoria was a main protagonist in that (it' being written).