Very busy but TONS of fun. Just get there early and DON"T LEAVE the park or you may not be let back in.
Funny thing is that line times are crazy because so many people are camping out ALL DAY for the midnight fireworks. Main Street will fill up with people by evening time.
We were there for 08/09 and it was so foggy you could not see the fireworks at all, you could "hear" them but that is about it.
The cool thing is that they open up "back stage" areas after the fireworks to allow the HUGE crowds flow better. We got to go from Main Street to Space Mountain back stage, it was kinda cool.