We went in August, and yes, it's H-O-T hot! However if you prepare for it, it's managable. We would arrive at the parks early so we could get a lot of touring in before the crowds showed up and the day got too hot. In the mid day hours we either did table service meals, or watch shows (in air conditioned theaters), or we'd go back to our resort for a dip in the pool (so refreshing!) and maybe a little nap. Then we'd head back to the parks in the late afternoon or early evening. It worked out great!
We found crowds to be very managable. We walked onto most rides and had little wait for many others. Only a few were long waits. We plan to go again in August. I could do without the heat, but with the proper planning you can handle it. Just remember to drink lots of water and take breaks when you need them.
Have fun!