We were there from 8/26 through 9/3. It was our very first trip during the summer, and I had braced myself for larger crowds than I was used to (other trips were taken either early Feb. or mid-Oct). It wasn't until a few weeks before we left that I realized that many southern schools would be back in session starting 8/15, more the following week, so this last week before Labor Day would be great.
I was not disappointed. I'd say that most lines were 10 minutes or less; many rides we just walked on. We did fastpass a few rides, but many times it was just to ride 'back-to-back' times. The only line that was truly long was for Soarin' in Epcot...ran about 60 minutes, but we used fastpass and didn't have to wait. When we returned to Epcot later in the week, the line was only 20 min...not bad at all.
Was it hot? Yes, but I live in Jersey and the heat/humidity was not really much worse than I'm used to during a heat wave here.
We'd definitely consider doing the last week of August again.