Out of four trips in October, the last in 2002, I offer the following:
The Good: It's Disney! But more important, it's a low attendance period, meaning light lines, uncrowded restaurants, inexpensive resorts, good bang for your buck. Also, the Food and Wine Festival is always delightful. Finally, Halloween on Pleasure Island is one of those special nights of the year. To me, anyway.
The Bad: The weather, if you go for the warmth of the Florida sun. We found the temperature to be all over the place. One day it shot up to 85 and I was unprepared, resulting in sunburn on the head and lots of sweating in blue jeans. Yet two nights later, it dropped into the lower 50's. Note, these aren't terrible things, they just require extra planning. Also bad: lots of attractions closed (e.g. COP, Timekeeper) and lot's of renovations. Note that most of the good and the bad is simply the trade-off for going during a light-attendance period.
THE UGLY: Stay the heck away from the period surrounding the PGA tourny.
That said, I'll personally never go any time other than September or October. Unless I go another month....