We have gone to WDW in September 11 times. It used to be a great time to go. Very low crowds, no wait times, etc.
Free Dining has brought in a lot more people. Our trip this past September (13-22nd) was our last September trip. The parks were very crowded, the restaurants were packed and the resorts were quite busy. We did not find the waits as bad as of of the previous posters there at the same time but, they were much longer then we are used to in September.
The heat is not an issue for us. We have gone so many times during the warmer months that it does not bother us. We know what to expect. It is quiet hot though.
Free Dining is an awesome deal. We used it one year but are now
DVC members. We can no longer get FD unless we book a trip off points. But, we won't be doing that with the added crowds.
If you have never gone to WDW during September before, it should still feel slower then other times. I find the people that notice the crowd increase the most are those that traveled there before Free Dining caught on.