Just booked for Sept 23rd to the 28th! I went this past September and had an amazing time. I thought my favorite time to go was in January when the weather is nice and cool and the crowds are so low, but all of the events and decorations really make September worth it! I even noticed a huge difference in the amount of cast members around to help and in the street entertainment. (My first time seeing DiVine, more activity from the citizens of Hollywood, etc.) Plus, it was my first time riding Splash Mountain in almost 20 years because it's usually closed in January.
I might be a September convert!