We went the first week of May in 2010 and it was HOT to me. 90-95 every day. We were expecting closer to 85. Even the locals were saying it was unusually warm. The May before that it was unusually rainy.
That said, if heat and humidity don't bother you, the crowds were AWESOME. It wasn't crowded, we didn't have long lines for things (for example we stopped to do tons of meet and greets on our way to get our TSMM fast passes and didn't have to join the "stampede" and got them for later in the morning no problem.) and I would LOVE to go back that time of year again, but DD can't be taken out of school again. Also, DH was miserable in the heat.
(We're going at Spring Break next year. Much better weather, but OMG, I am not looking forward to the crowds.)