We have gone in Sept twice now, and got married in WDW in Sept. We love that time of the year. It can be hot but not as bad as summer. But the characters are still out, the heat doesn't really affect their schedules since they work for 20 minutes and then get a 40 minute break (or something like that) so that they don't overheat. If you are concerned about seeing your favorite ones, consider planning one or more character meals. That way you get to see several at once and don't have to stand in lines to do it.