I haven't been in October, but we were there end of Sept. just last fall. I know rates go back up to "Regular" in Oct. (Sept. is Value season) and towards the end you'll have the
MNSSHP (Mickey's Halloween...) crowd there.
That said, you do have to consider hurricanes in Sept (actually, the official season doens't end until Nov.)....I'm gun-shy right now because last year was SO active in that sense...seemed like one after another. We drove through remnants of Ivan on our way to FL, and we actually arrived at WDW the day after Jeanne (which we rode out at the coast near St. Petersburg, thank you very much!

Never again...). WDW did a miraculous job of getting the parks back open and cleaned up. While riding the bus from our resort to the parks, you could see down some side roads (service roads) that hadn't been cleaned up, and WOW!...It made you realize they had probably been out all night doing clean-up! Anyway, getting OT here..sorry!
That said, the crowds were VERY minimal at the end of Sept.! We didn't have to use FastPass for anything, did things like Buzz Lightyear, Winnie-the-Pooh, Big Thunder Mt. Railroad multiple times in a row with very little or no wait. Epcot was a dream just because it's so big and can swallow crowds anyway!! It was definitely hot enough to swim (which we did every afternoon for our little down-time break). Still felt like summer to us northerners!
Fall is a great time to visit because school is back in and gererally you're between the summer-break crowds and the real holiday crowds. I think you really can't go wrong with either month.