We did the halloween party in 2000. We loved it. It was great to see the park all done up for Halloween, the characters are in halloween costumes (yup, costumes in costumes), and the best part is how not crowded the park is! We were there for our honeymoon and got our ticket for the party on the day of our arrival. It worked out perfectly for us. We got to our timeshare (we were staying offsite), caught up on some lost sleep, went out to dinner and then headed over to the MK. We got into the park and found there were practically no lines anywhere! It was perfect. We managed to do almost everything we wanted at the MK in just a few hours. That way we devoted the rest of our vacation to the other parks, and went back to MK on our last day and visited the few things that we wanted to do that weren't open.
I would definitely do it again, if we were back down during that time. It is great fun!