I've been told by many on here that it's really not too bad, with the exception of the day of and weekend of the 4th (3rd-5th), and really only in the MK. I'll be down there to, and am somewhat counting on this.
I'm also hoping that the fact that the 4th falls on a Sunday this year will pose a big benefit: Some people are going to be off work/school the week that ends w/ the 4th, and some will be off the week that begins w/ the 5th. That weekend will still catch traffic from both crowds, but I'm hoping it serves to split up the crowds during the weekdays of both weeks.
Expect MK to be especially crowded this year on 7/4: not only is the 4th July, it's also a weekend, and it's also an Extra Magic Hour day there. Tourism's up too... this could be a monstrously crowded MK 4th of July... but hey, that's half the fun right? (And from everything I've read, worth it, if you're willing to sacrifice some rides that day).
-Aaron