We have been at Christmas for several years now (it is the only time that both myself and DH can take vacation at the same time!

) Hey--looking at my signature line, I see we have going since 2000, every Christmas! Yikes!
It is DO-ABLE! And you can have a great time! Yes, the week between Christmas and NY is the MOST CROWDED OF THE ENTIRE YEAR at WDW. The week before Christmas is crowded also, but not as much. The crowds build steadily from the 18th or so...starts not so bad and gets worse and worse as December 25th approaches.
You can still have tons of fun, even in the dreaded X-mas to NY week. You must resolve to get up early, be at the parks at opening, make your ADRs at the 90 mark...use evenings for parades and fireworks, not rides! We went last year (after our Cruise

), Dec 27-30 OFF-SITE, no less. We have also flown in on Christmas day, and stayed the week-not-to-be-named

We had an absolute blast!!! Never waited in lines, bailed out of the parks before GRIDLOCK hit, and YES, it does hit, and it hits hard!!!!
I agree with the previous poster about using TourGuideMike. I subscribed this year (another upcoming Christmas trip, this time Dec 18-26, with my parents, brother SIL, all their kids, and mine!). His planning is RIGHT ON. I highly recommend subscribing to his service!