Have been in July (4-12) and it was VERY HOT and VERY CROWDED! Went last Aug 21-29 and both DH and I felt crowds were much more manageable. Yes it's HOT, yes you have to plan, take breaks, drink, drink, drink. But, I find a lot of attractions are indoors so you only have to deal with the heat walking to and from. Just plan on doing parades, outdoor shows and such at night. We don't do afternoon parades. The one thing we did do last year was wait 40 min for HS Musical. It was PURE TORTURE. I thought I would pass out. It was waiting in front of the hat at MGM on the blacktop without a lick of shade. Never again.
We are returning in Aug 19-26 this year, cause summer is all we can do/afford right now. The airlines to FL from NJ are great in the summer and the value season means we can stay deluxe. Plus, kids are in MS/HS and can't pull them out anymore.
My advice. Eat a light breakfast at your hotel, plan on at least one table service meal per day. Believe me you will need the break. We do one of two plans per day. Either get to the park at opening and be sipping drinks poolside by 2pm. Or we do a water activity during the day and hit the parks at night. Again, I can do this because my DD's are 12 and 14. Last year my DH and DD14 did MGM until close and then took the boat over to Epcot for EMH until 1am!! No crowds, cooler weather.
Either way, I say last week of August beats the rest of the summer hands down.
One more note...the rain. You do need a back up plan. We were at DTD for BBB last year before a PP party. It POURED. DTD was FLOODED. My girls were all "done up" for PP. We hailed a cab to get to MK cause I didn't want to wait for bus. The PP was miserable. It rained until 10pm. We asked at Guest Relations and they let us come back another night cause most of the attractions were cancelled or shut down. While the 1 hour showers in late afternoon are a welcome relief - 6-8 hours of pouring rain can put a damper. Be flexible and creative.
HAVE FUN - and maybe I'll see you there!