I thought I would chime in. We went in August in 2009 - last week of August. As others have mentioned, crowds were low, lines are short and bring a poncho or light rain jacket - you will need it! It rained every day, usually early afternoon. Usual pattern on our trip was a torrential downpour for 5-10 minutes, then just clouds for a while, then more rain, but just light stuff. Shopping or theater type shows are a good option when the torrential rain starts. The only downside to the rain is when there is lightning. If the lightning is close they do close down pools and waterparks. We had that happen a few times with us at Stormalong Bay - a bummer, but safety always comes first

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I concur on the laundry! It was a must for us (luckily we have DVC).
I have lived in Winnipeg all my life, and what I was not ready for was the unrelenting humidity. I can tell you I had never experienced humidity and heat like that before. As someone else mentioned, even after a rainfall there was no change in the humidity. As soon as I stepped out of the hotel, I was literally covered in a sheen of sweat which only disappeared if I went into an air conditioned space. It was not pretty

but there is air conditioning all over the World, so we could always cool down - it gives you a whole new appreciation for Spaceship Earth and Carousel of Progress (I still catch myself singing "Its a Great Big Beautiful Tomorrow" on occasion). We also brought fan misters with us we picked up in the camping section of
Walmart. Our kids got a lot of use out of them.
The other thing to watch for is camera equipment - whenever you leave the hotel you can have a significant condensation issue, especially if you have a DSLR camera. What I would do is about a half hour before heading out, I put my camera on the balcony with my camera bag and the camera in a baggie. It would solely warm up to the outside temperature and I could avoid condensation from the hot humid air contacting my camera that sat in air conditioned comfort over night.
Even with the Humidity (it deserves a capital letter!), we definitely had a blast that trip. Stay hydrated, bring your rain gear, and you will be good to go!