We went this past June during Gay Days and it wasn't horrible. We did Tampa/WDW just for a long weekend. It was hot, humid and it rained in the late afternoon/early evening but it was manageable because we take afternoon breaks and get to the parks for opening. We arrived at WDW on Saturday and it was just a Poly/Pool day. We did MK on Sunday and it was an EMH day (don't recommend) but we rode everything in Fantasyland that we wanted to ride by shortly after 10 am or 10:30, then took the train to Frontierland and grabbed FP's for BTMR, no problem. Now when the train got to the main gate - Wow, that was crazy crowded but we took our afternoon break shortly after lunch and returned for dinner at CP (ADR's were a must). I think if we had hit a non-EMH park we would have been fine. If you have a good touring plan I would say go for it. Even though its hot, its not as bad as it is later in the summer.
I have never gone for July 4th, but I understand that the parks have had to close the entrance due to being at capacity. To my knowledge that never happened when we were there for Gay Days weekend. And had I not known about Gay Days from the Dis, I would have just assumed that summer crowds had descended.