Count me in as another one who did it once and won't be doing it again, unless there is a very extreme circumstance (i.e., I won't be getting back to DLR for another 10 years and NYE is my only chance to go; someone I haven't seen in 100 years is in town and wants to do DLR on NYE and they can't go at any other time - that sort of thing). I can only speak for my experience back in Dec. 2000 - not sure if the crowds these days are worse or better (the whole AP holder dynamic has shifted over the years, and also now there is DCA, which can possibly eat up some of the crowds that would have been crammed into DL).
It was scary, scary crowded - especially on Main Street, because people stood there from something like 8 p.m. on until midnight, and wouldn't move. We were eating at Carnation, and we saw the crowds begin to build on Main Street, and by the time we left the restaurant, it was bad. It was really, really hard to even squeeze through the people to get onto the sidewalk, and then once we made it to the sidewalk no one was moving! One of my friends 'suddenly developed a sore throat' and had to head back to the hotel. I think she just panicked, and I couldn't blame her!! Finally, my other friend and I - who decided to stick it out - headed towards Tomorrowland, where we stayed at midnight.