We've been during Thanksgiving weekend, it was busy, but didn't seem any more busy than February President's Day week. Christmas week was a completely different animal. We were at MK on New Year's Eve one year when they closed the park because it was at capacity. We wanted to go back to resort for food/jackets, but there were a few hundred people at front gate waiting to be let in, so we decided to stay. Then we got stuck in gridlock on bridge to Tomorrowland, so many people moving in both directions that no one could move, trapped for 5-10 minutes not being able to move, that was insane.
Having said that, my parents go to WDW at Christmas almost every year. MK usually has morning EMH every day at 7am. My parents are there for 7am rope drop, ride a bunch of rides (they don't care about big thrill rides) before the crowds get crazy, then wander around, relax, eat, do less popular rides/shows.