September is absolutely my favorite month to go to WDW. After Labor Day the crowds (what crowds??) fall off to nothing. No lines, no waits, no hassles. It's wonderful.
The only time it gets crowded is on the weekends in the MK. That is normally the only time they have the fireworks and which ever parade they are doing at night.
It is warm enough to swim most, if not all, of the month. I like it really warm for swimming and temps in the upper 80's or low 90's are not uncommon for September.
The trade off for all of this is shorter hours. But you can accomplish so much more in the day, you don't end up feeling cheated. And, of course, EPCOT is still open until 9 p.m.
The weather is usually beautiful. Of course it is hurricane season, so you need to keep a watch out for that. I was there several years ago when WDW did have a tropical storm. It was no big deal. I spent a wonderful morning at Port Orleans sitting on the boat dock watching the birds and the fish. It was like I had my own private resort. For nearly 3 hours I never saw another guest. By that afternoon, I was back at EPCOT.
So give September a try. You won't be disappointed.
Roberta .... who's had 9 of her 13 trips to WDW in September.