The City Mini rain cover is awesome, and it can help you differentiate your stroller from others. I'm actually thinking about bringing our City Mini Single rather than our Joovy Caboose, because we already have the rain cover. I didn't buy it specifically for Disney though.
I'm having a hard time justifying $40 for a rain cover for the Joovy, since we don't live in Korea anymore, so I probably won't get a ton of use out of it regularly.
That said, it was really awesome to have a rain cover for our stroller when we went to RenFest last year. It ended up pouring, and we kept the kids dry.