It's terrible. Horrible. Cancel your vacation and stay home!
Seriously, due to work and school schedule, it was the only time of the year we could book our vacation. So we went about the same time you are planning for, but last year. We are
Disneyland AP/regulars and we hadn't been to WDW for over a decade. We were terrified that it would be horrible.
As a background, we're huge Disney fans and my family did a great job planning and sticking to a plan. Our basic touring plan was visit the EMH park one hour before opening, book FP+ for 10AM, 11AM and 1PM and schedule a sit down lunch around 12:30PM.
We would rush E-ticket rides that we didn't have a FP+ for. We would pick up other D-A ticket rides as the morning progressed, in between our FP+ reservations. After lunch, we'd do shows/parades and less popular attractions.
It worked out great. Our family was within the first 10 families of any ride we rushed. We felt like we got to ride everything we wanted multiple times.
The only day the crowds seemed really oppressive were 12/25. We wanted to spend Christmas @ MK, just like 70k of our closest friends. I think they shut the MK around noon. Our rush/FP+ worked well until about 11AM. Then the park was so crowded, we couldn't even move around the park. It took us about 20min to walk from iasw/Pan over to Liberty Tree Tavern. The whole are was packed and no one could move.
We thought we could "beat the crowds" and take a ride on the Liberty Bell. It was horrible. The boat was over loaded. People were sitting on the floor of the boat and we couldn't even walk around the boat.
Anyway, of all of our ten days around Christmas, there was only ONE day where the crowds really stopped us from doing what we wanted to do...