We were there last year Dec 20-23. There were definitely crowds, but it wasn't too bad. We got there early in the morning, hung out at the hotel in the afternoon, and went back to the parks in the evening. Also, we were willing to skip shows and parades to hit the rides when the lines weren't too bad.
The one morning that was VERY bad was when we went to Animal Kingdom on an "early hour" day. We were staying offsite, so we couldn't go in early, and then once we did get in the park was mobbed.
This year we're going Dec 19-22. We're staying at POP so this time we'll either get to the parks early to take advantage of the early openings, or avoid the early-opening park on that morning instead if we wouldn't get there until normal opening.