We went July 4th week in 2004 (checked in on the 4th for 6 nights). It was horrible in the MK on the 4th, but it was fine the rest of the week. Crowded, yes, hot, yet, but it was manageable. We hit the parks early, took a swim break, and hit the parks late. We did pretty much everything except the very minor attractions with no problems. On all our trips (we mainly go inthe summer) we have only waited longer than 30 minutes for an attraction once or twice. We hit the big rides early, use fast pass, etc. and always manage to see almost everything. It is definitely DOABLE!
I have never been Christmas week, so I can't compare, but I heard crowd wise it is worse, but weather is better.