we were there last August and the crowds were pretty manageable. There were long waits at the big rides; at one point Space Mountain was at 3 hours! but it also kept breaking down which seemed to cause the long waits...
I've never actually been to WDW in the summer time, and I've only been to DL in the summer -until next week that is!!!!- and from what my DH, who has been to WDW in July & August, it seems like WDW is crazier. I also found that DCA was always less crowded....We were able to make good use of our fast passes, so we were able to do everything....I have been to WDW during busy times on the weekend, and once for a day during February vacation week, so I have seen it busy there as well, but that was waaaaaay before fastpass.
My advice would be to use your fastpasses wisely and be patient.