We went last year from Aug 21 to Aug 29. There was never more than a 20 minute wait for rides (and that's without fastpass!). I know that on EMH evenings the parks got really busy just before the fireworks, but not horrible.
I will say...if you are going during the free dining, make sure you make ADR's. Every TS restaurant we went to were turning walk ups away. We made ADR's and I'm glad we did...otherwise we wouldn't have gotten to eat where we really wanted to.