We were there before the 22nd of Sept in '05, and it was nice. That day was incredibly overcast, and was actually chilly, even down in San Diego (my brother blamed me for the weather b/c we live near Seattle), and while that was odd, it also allowed for a big downpour close to park opening, which made everyone go and/or stay home for several hours.

Park was ours, all ours, until well after noon. OK not really, but that's how it seemed.
Rest of the day was pleasantly sunny, not too much heat, just enough.
Couple things were closed, like Haunted Mansion, but that was OK.
We're going back around that same time, this time for 4 days, this year, so obviously we don't think it's a bad time to go!