Two years ago, we went to WDW from July 30-August 6. We only went to the evening EMH at MK, however. We were there until right before it closed at 2AM. There seemed to be NO ONE in the park from about 10:30 onward. By the end of the night, we were able to stay on Space Mountain, Splash Mountain, and BTMRR each 5+ times in a row without leaving our seats. The park was almost deserted.
I would say that if you're willing to stay up that late, it is definitely worth it--at MK, at least. I have always been a night owl.
But when the park closes earlier, it is inundated with people. If you are going to EMH, I would suggest arriving there after one hour has gone by. This should eliminate many people from the long lines.