My girlfriend & I have been twice and both times it was early September. The only gripe I have about the off season is that the parks close early. I was looking forward to fireworks in the MK, until I realized that we had missed them and they wouldn't be having them again until the weekend (after we had already left). So, I'd suggest that you make sure your trip includes a weekend, and make sure you see the fireworks/Electric Light parade/Illuminations/Fantasmic while they're available.
Both times we went, we thought the crowds were managable. Sometimes the lines were long, but at other times they were quite short. Either way, we always enjoy the pre-show experience. The Imaginieers must spend an awful lot of time making sure that when we must wait, we can at least be entertained by our surroundings. That's one of the big differences between Disney and all the imitators.
Ooops, I'm rambling. Sorry.
OK to sum up our experiences in September:
Weather - Hot, but not unbareably, & not too much rain
Crowds - managable, not as many kids as at Peak season I'd guess
Park time - shortened, plan accordingly and you'll be OK
Off season rates -
Have fun!