Honestly,
There's two things I'll say about something someone else said:
The last few years we've gone but we've not entered the park until after 10:00 pm. We only go to EPCOT however.
We've never had a problem getting into the EPCOT parking lot (in fact July 4 there were more cars in Epcot's parking lot, including being parked on the incoming road coming into the park)
We've also never had a problem getting into EPCOT on NYE.
Now, personally I dont like WDW that much during that time but my wife likes to go.
Typically Epcot does very good fireworks at NYE but, this past year it was really poorly done IMHO and was outdone by MGM which was going on at the same time, and lasted about 5 minutes longer.
One thing I really hated was, there was a stage set up at the fountain (walking through that way is impossible, you have to go the long way around) and they played incredibly loud techno music all night long. Even the speakers through the park that typically play (background music) played techno music. When I left, I was completely worn out from the techno music.
I'll say this, if you go to EPCot at NYE, it's not going to be the typical EPCOT and in my opinion they go a little overboard at NYE and sort of ruin the magic of the park. Oh ok, they REALLY ruin the magic of the park.