Depending on when Christmas falls....maybe not that busy. It is VERY busy during the weekend prior, the week of, and the weekend after Christmas and New Years.
We went Dec 18-24 last year. It was not crowded until the 22nd. I think that if you plan(and plan and plan) and get there at rope drop, it won't be too bad. Either way, you will be at WDW and will have fun no matter what.
I went there last DEc around the second week and not too bad in the early morning. Around 5pm though it got so crowded you coulnd't even walk and it stayed like this the rest of the night. I was there for 4 days and it was like this the entire time. It took me over an hour to get a cup of coff and hot choclate to keep warm! I'm a true Disneyland freak, but never again in Dec!!