We've done end of August the last 2 years and going back again the same last week of August. Personally, I really love it. I like the heat... to me if I'm going to Florida, I want heat. It doesn't bother my family that much and it's not like we're used to it, by any stretch. I love being able to put on shorts and a t-shirt and not having to worry about getting chilled at night.
It is hot though, no denying it. But, I find with WDW there are so many opportunities to get a break from the heat, whether it's an ADR a/c'd restaurant, or an inside ride/ride queue, or an a/c gift shop, etc... There just really aren't a lot of times where you're out in the full blazing sun for an extended period of time. You need to stay hydrated and have a relaxed pace and then it's fine. We also bring a spray fan for my DS, which doubles as a cooling device and something to keep him occupied while we're waiting.
Crowd wise, end of August is pretty great, so that's a big positive as well But, I think you only start to get the benefit of decreased crowds in the 2nd half of the month, not the beginning.
The only negative for me is hurricane season. The rain in the parks doesn't really bother me (and there are lots of afternoon downpours), but the threat of a hurricane on a day you're flying causes tons of stress. Two years ago, we flew down on the day that Hurricane Issac was hitting the Florida Keys and I was stressed for 3 - 4 days beforehand that we'd get cancelled or delayed and that was not fun. Turned out we were fine, but it was close.