I've done it once before - in 2004.
Absolutely insane crowds, never seen anything like it. I remember our group had to pick one ride because we really only had time for one (3 hour wait for Test Track, I believe, saw parts of that queue I never knew existed).
You weren't exactly walking through World Showcase as much as being part of a massive crowd that happened to be moving in the same direction as you.
However, the fireworks show they put on was beyond words, especially the NYE tag, and it was worth all the crowds and insanity. I'll never do it again though.
So, get there early, be very, very patient, expect crazy waits, and enjoy the show.