I had always gone in Sept and last year we went in August for the first time. It was busier but if you read the guidebooks and have a plan for your must do rides its not too bad. Use fastpasses whenever you can and avoid EMH days. The busiest places were the table service restaurants due to free dining make sure if you want a table service meal you make a reservation.
It can't have been that bad as I am planning to go back asap in August,lol.