A great time to go!!
Our first 5 trips were in sept. No lines, no crowds, no wait times. You can get a PS the same day if needed, and at many places you still dont even need a PS reservation.
It is hurricane season, though that never stopped us. We usually go for 7-10 days, and it would maybe rain on us for an hour or so in afternoon, then just be humid. The humidity didnt bother us with no wait times. I can only remember being hit with a tropical storm once and it rained off and on all day. One day we had 70 mph winds, but within a hour or so, they were gone and it was back to the parks.
We have been very lucky though too. I remember 2 years in a row we would fly in right after a hurricane would hit, and fly out just a day or two before.