We went between Christmas and New Year a few years ago. It's one of the busiest times of the year in NY, as a lot of US tourists visit then. We'd been before in both November and January, and loved it. But Christmas was something else, I will never go again at that time.
It was absolute bedlam. Streets weren't just busy, they were absolutely packed. Nothing was really enjoyable as it was so busy. We took our 3 teens, and in some areas (Times Square, 5th Ave) we had to fight our way through crowds to just move about.
The Christmas lights, and windows were lovely. But you couldn't get near some of them. Tourist sights (Empire State, Top of the Rock etc) were manic, even if you'd prebooked.
On top of that, it was bitterly cold - about minus 14 degrees, with a windchill of minus 18. We had all the things we should - warm coats, hats, decent shoes etc and we were still freezing cold most of the time. My DD was in tears with the cold.
It wasn't all bad, we did see some great things, and had a good time in parts, but I will never go again at that time of year. We were unlucky with the weather being quite so cold, but it was just so busy. We are going again, but at October half term this time! Hoping the weather is at least a little warmer, and we see more!