We were there June 8-15 this year. It was HOT HOT HOT. I live in Florida so I'm used to this. But my ds has high blood pressure and the heat really affected him at Disney this year. It was crowed too. It didn't really affect us that much because we always get to the parks at opening, and take breaks in the afternoon, then return in the evening. But be prepared for Heat, Some rain/storms and some crowds. It sounds bad, especially if you are used to going during slow times, but it's not that bad. We however will choose to go only during the cooler months with my son now.
There is ALWAYS a way to have at Disney, so go and have a great time no matter when you go!