In 2009 we went the Wednesday - Sunday following the Cast Member parties. Our experience was great!
Wednesday and Thursday, we thought crowds were quite low, even though the parks were open from I believe 9 AM to 11 PM and there were two showings of Fantasmic! Granted, we had last gone in mid-to-late June, shortly before some AP blockout dates, so anything would seem nice to us.

We saw Fantasmic! on Wednesday, the second showing. I'm pretty sure we could've shown up 20 or 30 minutes before that second showing and got SEATS! It was completely and utterly different from June.
Coming in at 2 PM and seeing these wait times just blew our minds with how wonderful they were. 20 minutes for major attractions!
All predictions were that weekend crowds would be huge. I don't know if they would have been huge or not. Friday, it rained a little bit (more near the end of the day), and Saturday was rainy all day. So that really kept crowds down.
Wait board from Friday shortly before noon:
Saturday night at DCA was DESERTED. We were the only, or close to the only, people walking through much of the park. Toy Story Midway Mania was practically walk-on. (That said, I know that lately if it's rainy and crowds are low, Disney has been closing the parks early instead of leaving them open.

) Sunday, we left around 2 and crowds were pretty heavy (40 minute standby for Space Mountain, but FPs were available with a return time of 4:50-5:50).
But again, if the weather had been clearer, that Friday night and Saturday may have been packed. I heard that the following weekend was just TERRIBLE as far as crowds, and I'm wondering if a bit of that couldn't have been from locals skipping a rainy day and going during a sunny one.