We visited on December 25th and 28th, 2012 during our Christmas week visit. Christmas Day was amazing and very manageable. We arrived before park opening and the park was virtually empty all morning. Things picked up a bit in the afternoon but it was really not bad at all.
December 28th we arrived early afternoon and it was so crowded

the parking lots were full, and they had guests parking in the field next to the Renaissance. Tons of bodies, long lines, packed shows. We stayed away from the rides completely and did Dine with Shamu, the Christmas Marketplace, etc. A totally different experience from Christmas Day!
If rides are a priority, definitely consider getting Quick Queue for your visit.....its well worth the money IMHO (or stay at one of the 7 onsite hotels, and your QQ passes will be free

).