my husband and I have been to this twice in the last three years and we always enjoy it. A few things we've noticed based on our experience (we went once three nights before Halloween and once about two weeks before Halloween):
Crowds tend to be bigger as the month winds down - seems like people want to go to the party as close to Halloween as possible.
Crowds are smaller during the week than on weekends.
Crowds are larger when the MK first opens for the event, which makes sense because people with children want to get in, trick or treat around the park, and see the first parade and fireworks and then leave. We saw huge numbers of people exiting the park by 9PM. When we left at 11PM CM's were practically throwing candy at us as we left beacuse they had so much left over.
The wait time for rides is actually pretty minimal in most cases because a lot of people are trick or treating or watching shows or the parade. We found that hitting the rides during parade time was ideal and had the extra benefit of watching the fireworks start just as we were taking the plunge on Mount Splashmore (sorry...I mean Splash Mountain

).
Lots of people (adults, too) get into the fun and dress up - it really adds to the atmosphere and fun. Adults are encouraged to trick or treat (we do!).
I think this event is like anything else at WDW - you've gotta plan it out based on the size and ages of your family so that everyone gets as much enjoyment out of it as possible.
Hope this helps a little!