We like WDW in the summer, it's the only time we've ever gone.
We found the heat shocking when we stepped outside at the airport on arriving at Orlando but we got used to it quickly, within a day.
The crowds and wait times didn't bother us as long as we got to the park at rope drop and stuck to our touring plan. Sometimes even with a touring plan we got pretty hot and tired from all the walking in the heat. Every park has nice, cool places you can go to relax for an hour or two in the heat of the afternoon. Sometimes we'd go back to the hotel or to a mall to shop and then return to the park after 6pm. The parks are often still crowded in the summer from 6 - 9pm but we would always feel more relaxed.
In five trips to WDW all during the summer, I think we've only had 3 days or evenings when we either got soaking wet from a rainstorm or else had to miss more than an hour of touring due to rain. (plus one day and night stuck in a hotel sitting out Hurricane Charly, but that's another story). We didn't mind getting soaked (either in rain or on wet rides) because it actually makes you feel cooler afterwards when you're walking around in the heat (assuming you don't mind the feeling of damp clothes and squishy shoes).
We're always careful to wear hats, sunscreen and not stay in the direct sun for longer than necessary, to avoid sunburn and heat stroke.
When the kids were little we bought folding fans from China in Epcot and they used them to keep cool whenever we were waiting in a queue. They're cheaper, lighter and simpler than carrying around those water bottles with electric fans.