Going in August this year... We went in Sept last year and it was Hot every day. But with plenty of water (and a towel to keep our daughter out of the sun while sleeping in the stroller), it wasn't too bad. Most of the rides are air conditioned anyway, so you always get a little break in the cool(er) air.
The worst part, honestly, was my glasses! I have Transitions, and since the lines were extremely short at most attractions, they would still be "darkened" while going through the ques! Between my eyes adjusting to the dark, and my glasses being darkened anyway, I could barely see for a few minutes in the darkened ques. The worst one was PotC. I ran into three or four walls in there before I finally took off my glasses!