I am just your person!
I have never been to WDW in the summer, and probably never will. Our trips have been:
1st week October 1995
3rd week January 2000
2nd week November 2001
2nd weekend March 2005 (just us 2)
4th week Sept/1st week Oct 2005
1st week Oct 2006 (can't wait!)
All these times were awesome, chosen by my DH who is a former CM. He WON"T go in the summer, as bad as he misses it. Sept/Oct are typically still warm weather, so we swam, went to the water parks, etc. We live in TX so the heat didn't bother us too bad. It cooled off at night, and was nice. The
free dining plan has boosted numbers during this season, which was the purpose. But it's still not bad - just make your ADRs and you'll be fine. If school is not an issue, go in September - the crowds are much smaller as many schools (including ours) are having fall breaks in October. We knew lots of people just from our school that were there last year! We took the kids out early last year so we could stay longer, beat the crowds, and celebrate my son's b-day there. But this year we can't do that, so we'll fight the "crowds" in October.
January is another good option, as someone else mentioned. We had a freak cold front the year we were there and it actually snowed! Not a lot, but we were not quite prepared for the cold weather! The parks were empty, though, and we never had to make a reservation for anything (my kids were too little to appreciate CRT, but we had a great character breakfast with Pooh at Artist Point at WL - not there anymore, but it was fabulous! We just walked up and went in - practically the only people in the restaurant so we got lots of attention, esp. with 2 babies!) I would def. go back in Jan. if that were ever an option. Now that I am teaching again, I don't know if that could ever work out.
Good luck with your planning! There's a great absence-from-school letter at "the mouse for less" website!