We have been going the first two weeks in September for the last three years, and yes, it's not crowded at all. Since9/11, it's been even quieter. The park hours are shorter and you can only catch Spectro and Fantasy in the Sky fireworks occasionally, but, other than that, it's a great time to go. Now that EE is back, it's so easy to do a lot in one day. It is expected to be a little busier this year due to people taking advantage of the FTP, but not too bad. It's so early in the school year, folks don't like taking kids out that early. We are also going on the 5th because it is the weekend after Labor Day, and the crowds will have gone. There are two of us, and we never get PS's for anything. We don't like to eat breakfast out (not into character's), but have no problem getting seats for lunch or dinner with no wait at all.
Hope you have a great time and enjoy the World uncrowded.