I went this past year on the 4th of July, and it was crowded. We spent the morning at Animal Kingdom, where the crowds were not that bad. Then headed to Epcot in the afternoon for a little bit, the crowds were manageable there as well. During the afternoon, a bad thunderstorm came through and the parks cleared out a bit. We went back to the hotel for a rest during the storm and then were able to get into the Magic Kingdom around dinner time. At MK, the crowds were insane but the fireworks were incredible, so it was well worth dealing with the crowds. If you stay late, most of the crowd clears out after the fireworks, and you can get onto most rides without an extensive wait. MK was open until 2am, I think, so you can do a lot of stuff if you want to stay late.