HOT and HUMID are right! We went in sept 2010 and I was completely unprepareed for the humidity. I'd read about it, I'd checked weather.com, I thought that I'd experienced humidity before, etc. I was wrong. The humidity really got to us.
I'm sure this is no surprise for people from the south and from florida but were from so calif where it gets HOT (110+) but very little humidity. I'm pretty sure that for me personally I'd never go that time of year again. Now the good part is that there was no one in the parks. Epcot seemed empty to me. My boys went on test track 5-6 times without getting off the car. We're talking empty.
But I'm still not sure it was worth it.