that is the time we usually go (summer is usually the only time i can go because of school). the park is fairly crowded, but you can still see plenty if you make good use of fast pass. it's nowhere near as bad as the times we have gone during xmas or spring break, especially on weekdays.
We've visted in August a couple of times. Early mornings & after dinner is the best time to visit the parks. Afternoons were always crowded especially at MK. During our visit last August, it rained daily and lasted a few moments to a few hours.
I went 8/13-20 last year & all the rides were walkon. Never used fastpass! The only time we waited for a "ride" was a Disneyquest. The heat is not unbearable if you are not in the parks in the afternoon. We arrived at park opening left after lunch then returned to the parks at night.