It can be done and enjoyed. I have many times, including both the fake Milennium over 12/31/99 and the real one a year later. ... You just have to be realistic. Crowds are gonna be insane and you can't think 'I'll go to the MK at 1 p.m. and ride whatever I want.'' ... If you take the visit slow and see the entertainment and the attractions that never have waits like HoP or the amazing animal exhibits at DAK, then you can have a great time. But if you insist on riding Splash Mountain or Peter Pan's Flight no matter what, then you'll likely not enjoy yourself.
I've actually been toying with the idea of going back up next week just to do NYE at TPFKaTD-MGMS as while I've done EPCOT and MK many times, I've never done the Studios for NYE and they're open until 3 a.m. with EMHs! I am pretty sure I won't, since I'm not fond of the crowds, but who knows?
