HOT and HUMID! IMO, very uncomfortable. Despite that, we are going in Sept. too (and have been there before). Lower crowd levels make it worth braving the heat. Be sure to drink a lot of water and take advantage of airconditioned attractions. We also hit the parks early and leave by about 2pm. We go back to the hotel, swim, nap, and shower for dinner.
Honestly, I don't think the mister fan is worth it. It's so humid that the water just sits on your skin. Be sure to use lots of sunscreen and reapply frequently. Hats are a must too, especially for little ones. We also bring a change of clothes for each child. That way if they get wet (one a ride or in those areas where the water squirts out of the ground), we have dry clothes to change into. The wet clothes would be we all day in the humidity.
Bottom line: keep hydrated and cool off often. The heat can sneak up on you fast!
Have fun!