We have gone 2x in Sept.
The first time, it was hot, nearly as hot as August. Very low crowds. We were staying off site as this was not really a "Disney" trip. We did SW, US, etc.
Last year, Sept was pretty comfortable temp wise. It did get kindof hot mid afternoon. The humity was low. By 5 pm or so, it was quite comfortable. The parks were crowded (we were at WL). It might have been because that was when the FTP was offered. I'm not sure how full the resort was. There were always lots of people around, but it didn't feel "crowded". WL is one of those resorts people like to visit when they are not staying there.
Over all, Sept is one of my fav months to go.