We went Early Sept 06 then Late Oct 08.
Sept had less crowds but A LOT of humidity!!! Daily rain storms, but lines are almost non-existant (at least for us!)
Oct had bigger crowds (less than summer I'm sure) and was perfect weather! Shorts during the day and at night. Swimming during the day....at night it could get a lil chilly, but we were still in the pool.
There seemed to be a lot of non-USA folks there, and as someone explained it to me, it's a heavier time cause they vacation then. Don't know if it is true or not!
Avoid Columbus Day weekend as it is more crowded!