This year will be my fourth visit over New Year's. It is crowded!! We avoid the parks on New Year's and just have a nice dinner and watch fireworks from outside a park (Epcot's from the Boardwalk, MK's from the Poly beach, etc.) Make ADR's far in advance and the whole week get to the parks when they open and leave by lunch when they're swamped. It is definitely the most croweded time I have witnessed in WDW and I've been in every month but October (about to go!) and June. If you plan and get to parks early you will be fine. It will have calmed down a bit on the 7th/8th but by then you will be dealing with marathon crowds.