We've mostly enjoyed the rain. On our first trip, it seemed to be "Well, it's 4pm, here's the half hour of torrential downpour!" People fled the parks in droves while DD and I had a ball running from cover to cover to get where we wanted to go. We were drenched, but it was warm and we were just roll-on-the-floor laughing at how we looked. On another trip, it rained lightly - not enough to stop those of us who go out in snow (lol), but enough to thin the crowds considerably. On our last trip, it POURED just as MGM was closing, so of course the entire crowd was now waiting at the bus stops, which have only minimal rain cover. We had long waits (They did start sending extra buses, but really we were all soaked after 2 minutes) but it was kind of nice - everyone pulled together to share as much of the cover as there was, cover other people's kids with umbrellas, etc. I admit to being a little amazed at not seeing a single negative incident.... maybe there was a little extra Magic in the air that night.
Throw on your rain ponchos or jackets and enjoy!