I have seen the MGM fireworks, the Epcot fireworks, and the Pleasure Island fireworks on three different NYE nights. I think Pleasure Island shows the same fireworks they do every night. Not terribly impressive.
MGM shows their Fantasy in the Sky display (the ones they almost never show any more). We watched them from Hollywood Blvd (near the front of the park), and when the MGM fireworks were done, we were able to turn around and see the airal portion of the Epcot fireworks in the distance.
Epcot shows their usual show, then they show the additional NYE fireworks. The Epcot NYE fireworks are really, really intense. I just asked my sister if she could remember what music they played during the NYE fireworks, and she said "I don't know... it was so loud, you couldn't really hear the music." Again, if you are near the International Gateway, you can turn around and see the MGM fireworks at roughly the same time as the first part of the Epcot fireworks.
All of the parks are crowded NYE. In MGM, it was packed shoulder to shoulder on Hollywood Blvd. I saw the NYE fireworks at Epcot form an Illuminations cruise, so I can only tell you that the park looked incredibly crowded.