We've done WDW twice in August, got hit by Hurricane Charley the second time. I think the crowds are pretty manageable in August, especially the later in August you go. I'm not a fan of the heat, and we'll probably not go in August again. Since we come from an area that is usually not that hot and humid, we found it to be pretty uncomfortable and an abrupt transition. But like others say, there are plenty of things to do to beat the heat, and since we found the crowds to be manageable, we were not queued up outside that often. I definitely swam quite a bit.
Really looking forward to our first trip home this year in October.