We went to
Disneyland once on New Year's Eve.
"It'll be fun! It's Disneyland, it's NYE, it's outdoors, it's free (we had APs at the time), what fun! Let's do it."
The simplest summary I can give is: NEVER AGAIN
You literally could not walk on Main Street. At the end of the night, there are so many people exiting the Park, the trams are not running. They line the route to the parking garage and you WALK there. The special festivities were nice, but they were shared with 100s of 1000s of our newest closest friends who were literally squished up next to us (and others on EVERY side of you) so that you could not move an inch
Did I mention, "NEVER AGAIN"?
But if you are as huge of a Disney lover as I am, maybe you have to do it once, just to know that you will never ever do it again.