It depends on when in December you are going. We were there last year Dec. 30 - Jan. 3. The first 3 days we were there were really crowded, but since we had Express staying onsite, it didn't affect our fun. The streets were full. It took a while to navigate through the parks, but we never had to wait in line more than 10 or 15 minutes - even with the extraordinarily large crowd. There are lovely decorations, Whoville in IOA, ice skating, fireworks, etc. Everything was just beautiful. Earlier in the month is probably not crowded, but I don't have personal experience with that.
Nan