We have been twice in October. Last year it was hot, but I didn't consider it opprossively hot at all (that is, until we had our costumes on, lol) It gets quite hot where we live, so I am used to heat, and in fact, love to visit it in Florida--but of course, we don't go in the middle of summer or anything--that is when they really get what I would call "oppressive" or "unbearable" heat.
As for crowds, it's a fantastic time to go IMO because the crowd level is relatively low (not peak crowds or anything) but the weather is warm enough to enjoy everything. It is more crowded as you get to the days closest to Columbus Day (before and after) but not so much that I found it a problem, nor would I avoid those dates. I would certainly go again at that time.