September is a great time to go. The crowds are hit and miss. Just when we think we booked the perfect time to go with no crowds, something crazy comes along and the parks are packed.
Just plan on having a great time. I do not think that it will be bad at all. We just went in July (4th of July week) and we were expecting it to be insane. It was very crowded, but nothing like we thought it would be. And, the weather was not as hot as it was supposed to be either! You just never know...