This has got to be one of the best value things Disney does. Done it 3 times and would do it again if we ever go in October again. A lot cheaper than a day ticket but you get almost as long, I think ticket says around 7pm but last time we had no problem going in at 5pm and they picked us out on entry to sit on the Onmibus and wave to crowd before the parade. Earlier in the month the quieter the rides, closer to Halloween it can get quite busy.
First time we did it we just stayed on the rides like BTM with only a handful of people waiting to ride.
Dressing up does add to the evening but not essential.