We were at WDW last year over the 4th. Got to MK at rope drop on the 3d and it was pretty packed by noon. But do-able. We had planned to stay for the fireworks; however, my mother and DD were worn out by 4 pm and outvoted me--we left the park and headed for the resort. And by the way, there were terrible thunderstorms that evening, which may have affected the fireworks anyway? Not sure but it's something to keep in mind, anytime in July.
We were at HS at rope drop on the 4 th. This time, we had dinner reservations at Hollywood and Vine, with the passes to see Fantasmic. I think both are slightly overrated, but what I didn't realize is that they have nothing to do with the fireworks display. By the time Fantasmic was over and my mom and DD were ready for "home," we could hardly get down the street. I suggested we might as well stay but was once again outvoted and so haven't seen any 4th of July fireworks!
To sum it all up, in our experience it is possible and even enjoyable (we had a good time most of both days) but you need a lot of stamina. My DD and I will be back this 4th, and we've decided it's EPCOT's 4th fireworks or bust! We're planning on arriving in the afternoon, but I see from other posts here that it shouldn't be a problem (right?).