We've done NYE at WDW several times, and it is a blast! That being said, it is also the most crowded time to be there. The biggest mass of humanity I have ever experienced was trying to leave Epcot after the midnight Illuminations on 12/31/99/-1/1/00. It was shoulder to shoulder with the entire crowd moving as one mass toward the exits. I was so amazed that we only waited about 10 minutes for a bus back to OKW!
Be forwarned that if you go next year (this year is sold out), be prepared to get to the park of choice for your NYE celebration very early. We stay away from the MK that night, but have always done Epcot. On non-millenium years it hasn't been that bad, but you do need to get yourself inside by mid afternoon. Last time we had a handicapped vehicle with us, and the person in the cart was able to reserve a prime Illuminations viewing spot near Rose and Crown, and her whole party (us) was able to join her. It was fantastic, because we had no crowds around us as we watched the amazing NYE show!