Hi Dana!
Mickey's Not So Scary Halloween Party is one of the best event's at Walt Disney World! I have been on various occasions with and without the kids and each time has been a blast! Halloween is my second favorite holiday besides Christmas, so the party is a great way to celebrate! The Cast Members and Characters are in special costumes and there just seems to be a really fun atmosphere in the air!
The party "Officially" starts at 7:00 PM, but guests are allowed into Magic Kingdom at 4:00 PM. All other park guests without a ticket to the party are asked to leave at 7 and are not allowed on the rides or to trick or treat. Party nights are restricted to a certain number of park guests so sometimes they may sell out. The nights I have been, most rides were walk up with little wait.
The "Boo to You" Parade is one of the best at the park, if not THE BEST! I can't get that song out of my head now... Thank you! Seriously, the parade is wonderful and the Grave Diggers ROCK!
If you can afford to buy the tickets, I can vouch for the party and say it's a definite must do. I would recommend to have a resort day the day of the party just to save one of your park days and use it during the rest of your vacation.
Thank you so much for asking the Mons Panel! I hope you have a wonderful day!