We did in 2009. Crowds were great. It was still crowded in Fantasyland during the middle of the day, but that is to be expected. We were able to walk on most rides, though (and did MK during EMH so that was no problem). Most attractions didn’t have more than a 30-minute wait. It was great.
It was really, really, REALLY hot, though. And humid. It rained a little bit some days in the afternoons like it tends to do during the summer in Florida, but the rain didn’t really affect us. It didn’t last more than an hour or two.
It was MUCH less crowded than the rest of the summer, though. (Well, I don’t really know about that year because we didn’t go during that summer, but we have done June and July trips in the past and the crowds were terrible, but for Labor Day, most of the summer crowds were gone. I think most kids were back in school.)