We were at WDW the last week of Sept last year. The crowds were low. I think we had 2 waits of about 20 minutes, but that was it. Down side is that it was HOT & HUMID. Fantasyland was busy as always, but the rest of the MK was easy to navigate. We had to break each day, as our strength was sapped, and enjoyed the pool and A/C in the room for a nap. However, due to the free dining plan, the table service reservations were hard to come by in some locations, so plan ahead or you will miss out on the more popular places.
We'll never do that time of year again, because of the heat, but if you don't mind, it was really the only negative of that week.
PS, we were surprised at the amount of kids there. More than we thought. our then 6 year old, is on a balanced schedule, so her fall break fell during that time, so I wouldn't be surprised if many others were similair, plus many non US residents.