Even with the crowds, you can still have a great trip during this period. I have been over New Year's several times and my daughters would prefer this time--parks sometimes open well past midnight (MK open on Dec 30 from 7AM - 4AM, and then on Dec 31 from 7AM - 1 AM, including EMHs)--then again they are night owls.
All in all, you can have a great trip if you go with the right expectations AND a plan!