We took our first Aug trip this year. We went the very last week of Aug., schools in Fla. were back in session but it was still before Labor Day, which is when our school system starts.
We have only ever been in Jan. before this. It was more crowded than it usually is in Jan., but I would not call it packed. With careful planning, arrival at rope drop, and wise use of fastpass, we were able to do everything we wanted to do without waiting.
It was definitely hot, but it was about the same as it was here in VA, so I guess we are used to the heat. It added a layer of planning to avoid being outside during the hottest part of the day- unless we were in the pool! There were more rainy periods in aug. than Jan., but the rain was much more short-lived. Rain never stayed too long- in Jan., a rainstorm usually lasted much longer.
Given absolute choice, Aug. would not be my first choice, but given the chance to go in Aug. again, I would not hesitate.