It's a wonderful party,and I would absolutely do it again.
We went this last October with my children 2.5 and 15 months at the time, and my MIL, SIL, and BIL and there three children. Since my children were so little, they weren't interested in the trick or treating. So while my niece and nephew trick-or-treated, we walked on rides in Fantasy Land.
My children were afraid of the parades so we also used that time to go on a few rides.
It's a wonderful experience for all ages, and well worth the cost.
I think Baloo has a wonderful, extremely informative post that she writes out about the party.
I just wanted to add that I had my children in very light costumes. It was in the 90's that night (October 20th), a little unusual from what I understand. But it worked for us. I had two little Peter Pans. We did change clothes halfway through to pj's (again, they are so little , it didn't matter to them).