As to the question of busy...
I'll generally agree with the previous posters, the parks don't seem to be that bad, but the restaurants can fill up - especially character dining!
There are a few tips to make it seem less crowded.
If at all possible, go just after Labour Day. That's when schools start back in Canada (and some places in the U.S.) - going the week after Labour Day virtually guarantees that every kid in North America should be back in school. Sure, some parents will take their kids out for a week very early in the school year (that's what we did), but most don't.
Book your ADR's for the EARLIEST possible seating for Lunch and Dinner. Sure, you might have to eat lunch at 11 am, or dinner at 4:30 pm - but the restaurant will be a lot less crowded. It takes a little planning, but it is workable for most families.
Additionally, by eating at 11 am, you are getting away from the rides in the park just as things are getting busy. Of course, this means being at the park as soon as it opens.
I should say that all of this info is based on a Sept. '06
Free Dining trip - it may have been busier in '07.
Our longest wait for ANY ride that trip was about 20 minutes for the Speedway (and that was our own fault for trying to get on at 10:30 am)
Seriously, we were doing rides like Test Track and Soarin' back to back with no waiting - simply by starting early, leaving for afternoon breaks, and coming back to the parks at about 6 pm for a couple of hours AFTER our 4:30 pm ADR. At 6 pm, everyone else was trying to use their Free Dining in the restaurants, while we were enjoying line-free rides!
If you know some of the tricks, doing Disney in September should make you feel almost as if you own the place.
