September is one of our favorite times to go now. We usually went in the summer, and September was just awesome compared to Summer.
While you may have more rain, the crowds were basically non-existant. We walked on almost everything - we rode Splash Mountain twice in a row before more people were waiting in line.
It is still terribly hot and humid in September, and you may have to contend with "Love Bugs". They don't hurt you, but there were on everything - benches, doors, cars etc.
So, even though it was still hot, we had a great time since their wasn't large throngs of crowds - I suggest September to family and friends now.
We are trying out October this year (DH wanted to go to WDW when something was going on, so we are going to F&WF and
MNSSHP), but I think September will always be tops on our list.