We went for the two weeks that sandwiched Columbus Day in 2003. Although we did notice the increase in crowds over the holiday weekend (started about the Thursday before, tailed off by Tuesday), it didn't affect us too much - after all the October crowds weren't all that big in the first place. Probably added 10-15 minutes to the average wait time for rides at the very most.
However, there were a few things it did affect. Because it was off-season, there was still only one showing of Fantasmic and so everyone tried to get into it. Result - the theatre completely filled up with an hour and a quarter still to go before the show. And I mean FILLED - not even standing room was available. So we skipped it that night and went back on our last day once all the crowds had gone home - you could still get a seat with 30 minutes left (although one way off to the side).
