We've gone in September during 3 different years....and here's how the crowds stacked up:
September 2004. 9/9-9/14 Hurricane Frances was on the outs and Ivan was looming. NO CROWDS. I mean, we could walk onto anything without a wait....Splash Mountain in the middle of the day, etc. Towards the end of our stay, it picked up a little, but the most we waited was 10 minutes for anything.
September 2006: 9/16-9/20. Free dining, so a bit more crowded. Certain rides were walk on, and certain rides had maybe a 20 minute wait during the middle of the day.
September 2007: 9/12-9/18. Free dining. Crowds were much bigger this time. Unless you got to a park at rope drop, you would wait for the headliners.....like Soarin (up to 1 hour wait) EE (up to 1 hour wait) Splash (up to 45 minute wait) etc. Fast passes for rides like Soarin' would be gone by 5pm. By mid afternoon, all of the rides had some sort of wait. In the early mornings and late evenings, not so much. DH wanted to ride BTMRR during nighttime EMH....we waited 50 minutes for it. Definately more people in the parks, and the restarants were jammed. After reading a bunch of threads about people's experiences last year, I've found the people who report "no crowds" are the people who arrived at rope drop, toured, and then left to go back to their hotel in the afternoon. DH and I tend to get to the parks at around 9:30am or so, and then stay til close. So, I see the crowd fluctuations throughout the whole day.
As always, anytime you go, have a good touring plan and ADR's made. It should be fine. I said to DH, "I always came as a child during Easter week and waited for 2 hours for some rides....so anything less is cake!"