I agree with what everyone has said so far. Crowds are very manageable at the end of Aug, beginning of Sept. And since it is a non-peak time for most of the resorts, there will definitely be discount codes offered (we have gotten them 2 of the last 3 years).
The one thing that people have neglected to mention is the extreme HEAT!!!!
As well as the oppressive HUMIDITY!!!!!
I have been to Disney approx 30 times, however I went for the first time during the month of August 3 years ago. Following that trip, I swore I would never go again at that time.
Last year, due to the fact that it's the only real time I can bring my twin 13 year old boys during a non-peak time without taking them out of school, we went again. Following that trip, I doubly swore (with things I cannot repeat here) that I would never go again at that time of year.
Needless to say, we are scheduled to go again this Aug (18-25). I told my wife that it's kind of like childbirth. It's horrible when you go through it, you swear at the time you'll never do it again, but after some time you forget the pain and let your guard down.
All I can say is that you had better be prepared for extreme heat and humidity (it's usually in the high 90's during the day, and still into the high 80's well into the evening). A short walk at 10pm in the evening will cause you to break out into a sweat- forget about it at 3pm.
May God Help Us.