Hermasil,
Judging by your last couple of posts, you're living my life!
Our "traditional" times have been early June because both the wife and I have business around Georgia at that time, and then a personal trip in either late October or early November. Last year, we went from September 20-26. This is one man's opinion, and nothing more but...
I've never had a better time. Reason: prices are extraordinary on hotels and rental cars. Airfare, too, seems to be lower. The parks are delightfully uncrowded. To give you an idea, the Wednesday of the trip, we went to MK. We'd never been on Splash Mountain because it didn't interest us enough to face the hour-long lines in June. We walked on. Twice. Space Mountain was a walk-on, something I've never seen. We waited for nothing in EPCOT but Test Track (I think TT has a line when the park is closed). MGM had five minute waits on R&RC and ToT.
Other things: we were able to re-arrange PS's or get new one's practially on the fly. Friday night were were able to sit on the deck at the Rose and Crown for Illuminations (great view, marginal fish). I can't say enough.
Outside the parks: On Monday, Typhoon Lagoon had an empty parking lot. It was incredible. We'd avoided water parks in the past because of the rediculous crowds. This was unreal. And we're big Pleasure Island fans, partcularly of the AC. Enough people werw there to make it exciting and "club like," but we always had a seat and didn't have to elbow through crowds like we do in June.
The weather, also, was incredible. We always have the problem that I hate crows, but Amy hates cold. So our October/November trips were always hit or miss. When it's 65 outside, to her it's not Florida. Not a problem in September. The average daily temperature was 90. She was never cold. It was too hot for me, but no worse than June. And there IS rain, but it didn't bother us. Assume you'll see rain every late afternoon for an hour or so. Prepare for it and use it: It's great to take advantage of other's desires to not get wet by taking a walk in the rain and entering any facility you wish with no line. And there's something special about rainbows over the Lagoon in EPCOT.
Yeah, we liked September. The downsides are few, in my opinion, but here they are: There's a lot of refurbishment going on (there were large tarps covering half of the Haunted Mansion and most of the main building in the Canadian Pavillion last year). Rides can and will shut during this period for revamping (Maelstrom in Norway was not operating, and this time of year it seems like COP and Timekeeper are never running). The hours are shorter (although it didn't bug us as we were able to cover so much ground in less time). And there's the little disclaimer to all the happy stuff I wrote above: It does not apply to MK on Saturdays. Looked like a 4th of July there last September. It's the only day of the week they show Fantasy in the Sky fireworks, and I think half of Florida shows up. But we avoided this by spending the day in EPCOT/MGM, then sucking it up and heading over around 7 PM. And nothing is neater than roaming MK after closing on a Saturday night. Keeps you out of the exit rush, and it's quite beautiful.
That's my two cents. Hope it helps. I wish I could join you this September. We'll not be going this fall, the first time I've not been between Labor Day and Thanksgiving in something like eight years. But it's because we'll be welcoming our first child around September 21, which is a little Magic on it's own. Enjoy!
Pat
Are you having a ds this time or another dd or will it be a big surprise? I'm expecting ds#3... Still in shock over that one!!! 
Ughhh....I don't think I'll be getting another one either. Guess someone wants this one to be a suprise!! 
