All three of our trips have been the last week of August and we're going again this year. First time we went was 2007 and the parks were not crowded AT ALL! Then we went 2010 and 2011, and I think more people were onto the free dining thing so there were more crowds, but still nothing like what I hear for other times.
It is very hot, but I can deal with heat if I don't have to deal with crowds too. We've always done everything we wanted and never waited too long for things. I refuse to stand in a 60-minute line or more!
Once, we got to walk right on to a waiting raft at Kali River--no wait whatsoever. When we came around, still no one waiting, so we got to just stay put in our raft and go around again. This time around there were a couple people waiting, so we had to get out, walk aside so they could get in, and then get back in the next raft. We rode three times without waiting more than 30 seconds total!!! Rode 5 more times that day because the line was always so short. And this was August--when it's hot--when you get wet and cool off!!! The kids were in heaven!