Immollohan,
We were there late last Sept (21-28) and will be returning at the same time this year. (20-28) We loved late Sept.
It rained each day but only for 1/2 hour (one day it rained an entire hour HARD! but we were perched under an awning in Italy at Epcot and with a nice glass of wine and Pavoratti (SP?) singing to me it was one of my special memories actually.
It wasn't too hot, and the crowds were quite thin. With a little planning and research we hit the right parks on the right days. We never had more than a 15 min wait for any ride and frankly those were rare.
There were love bugs to contend with in late Sept. (little bugs flying around while they are maiting...attached at oposite ends....my son said "mommy, look! That bug had a head at each end just like CatDog!" they look like lightning bugs (fireflys to the rest of the world) with no light. They don't bite and we found them to be a small bother at best.
I have never been there earlier in Sept, so i don't really have a frame of refference, but that's my take on late Sept.
good luck - lori