We were there this last Sep for the free dining. We were so amazed at how low the crowds were...never seen anything like it!
It was HOT, but here's what we did: we got to the parks at opening, stayed for lunch, then headed back to the resort to swim. The kids swam for at least 1-2 hours. It was HOT from about noon-4:30. We then went to a different resort in the evening. The evenings were just wonderful. Very warm and comfortable.
All year long, there were predictions that Fall 2006 was going to be a big hurricane season. Well, they were wrong! I know when I was planning our trip, many people on the DIS commented that they weren't going in Sept because of the predicted hurricanes (the free dining came out on 4/1/06). I'm so glad we went in Sept.
It rained our first day there. The rain started when we got into our resort room. It didn't stop until early the next morning. And it wasn't just a drizzle...it poured!! We went to MK anyway. We still had a blast! Unlike last December when it rained during our trip when we were wet AND cold.
We said Sept is now our favorite time to go.
HTH