I was there from the 30th to the 6th and it was really busy.
A helpful hint would include going the extra magic a.m., but especially p.m. hours if you are resort guests. Though they closed the MK at noon, I phoned over at 9:30 p.m. from my room, and they said they had been letting people in for the last hour and a half, so I went to MK at 10:30 p.m. on 31st and was able to ride Buzz Lightyear twice, before making my way to the railroad end of Main Street and found a spot to wait for the fireworks. It was PACKED, but very festive and very worth it. Left the park after that, and was in my room at SS by 1 a.m., and that was with my mom using a rented ECV from an outside company.
Well worth trying to go to MK on NYE. It was my first, and the fireworks were THE best I have ever seen, and I've seen a lot. They had them at both sides of Main Street, and when the countdown began, each number was a firework in the sky. The then did fireworks at all clock points surrounding the guests to the song "Auld Lang Syne". Fortunately (and I'm not sure if they do this every year) but on the 30th when I was doing the Extra Magic hours from 1 a.m. to 4 a.m. (was able to walk right on to any ride I wanted), they had the same fireworks (with the countdown, "Auld") the night before. I was able to see them twice!!
Weather is not guaranteed (it was cold the week I was there, with a windchill in the minus celcius at night), though it was nice the week before I was there, and was going to be nice the week after.
With seniors who have ECVs, sometimes you don't have to wait in lines as long (depending on the attraction). If you have kids and do the EM hours, then obviously naps are in order. Allow yourself a couple of days at home before going back to work/school. You'll need to get caught up on your sleep.