It is crazy busy. But, it's doable. You have to not be freaked out by swarms of people. You have to use Fast Pass to your advantage. You need to do either "commando style" with lots of park hopping to avoid the worst times of day at the parks, OR just understand you won't see everything and be leisurely.
I'd recommend getting PS for sit down restaurants as much as possible. The counter service will be insane.
The lights and decorations will be beautiful!
I don't know if you have kids and the ages, but that may make a difference.
When I was growing up, we went to DL numerous times around Thanksgiving and x-mas. That's when family would be in town and want to go to DL. It was crazy busy, but we still had a great time. And, it was much quieter at night than during the day. Maybe more people doing day trips to DL instead of longer trips or more kids pooped out during the holidays. Though, it's been over 15 years since I was there during a true holiday time.
Last year I was at DL and DCA the Saturday and Sunday before Thanksgiving and there were lots of people, but the rides were still pretty good. Fast pass was needed, but not sold out. Counter service at DL was really difficult but over at DCA was fine. Actually, we went over to DCA late afternoon, and it was almost empty! We walked in, around 5:00 and got fp's for Soarin' for 9:00pm. Then DD and I went to princess dinner at Ariels and DS's and DH went looking at the rides. They ate a bite and wandered around. We were then able to go right on Screamin' without a fp. Anyway, sorry for the tangent, but I was trying to let you know that even during the very busy times, it's possible to still have fun and see just about everything. Timing is key. We went on alot of very popular rides later in the evening and had no troubles. (The kids rode both Buzz and Star Tours two times in a row right before closing.) The weather was gorgeous while we were there. We were there only for a quick trip as we were headed out to palm springs to be with the grandparents for Thanksgiving weekend.
