I've been there for several New Year's celebrations (including 1999/2000). They're generally very crowded - for 1999, the park closed its gates for a while (no new visitor allowed in). But... this year, all bets are off. From what I understand (I have a friend who lives 20 minutes away and goes regularly) it's been really quiet on most days, so NYE will probably be less crowded, too.
Good luck.
Gary