I've gone several times during the end of September/October timeframe.
Although other posters have said it's not hot at that time, please be aware that it can be very hot, in my experience. In 2009, I was there the second week of October and we had record-breaking heat several days in a row (90+ degrees and high humidity). It's not always like that, but it can be. (Sorry, don't mean to scare you!)
Plus, that time of year it can be pretty rainy, because it's hurricane season. I was at WDW during Hurricane Wilma in 2005; it was a low-level hurricane, so they only closed the parks for about six hours one day, and we actually had nice weather after that -- but be aware that it can rain at least a little every day.
But the trade-offs are lower crowds, the Food and Wine Festival, and Mickey's Not So Scary Halloween party. It can be a nice time to go, but be aware that the weather can be pretty brutal.