This past August was our first summer trip...we spent the last week of August, 8 nights, and left WDW on the Saturday of Labor Day weekend.
We had always gone in either Oct or Feb, and were already in the mindset that it would be very hot/humid and very crowded...wrong about the crowds!
Yes, it was hot and humid but with proper planning it never got to us. We took time to relax by the pool, and it seemed like there was always a place to duck into some A/C to cool off. I even carried a light sweater because I found it too cold in some places.
We dodged the afternoon rainshowers with no problems, and often it would rain briefly in one park but not in others. The rain was pretty hit or miss.
All in all, we found it a wonderful time to visit as the crowds were light and the parks were open later than the Oct/Feb timeframe we'd been going. If anything, we got to do so much more in the parks on this trip than others thanks to that.
I wouldn't hesitate to go in late August again.