It's something any Disney World fan needs to do once. Same goes for the Christmas Party.
Halloween is SO much fun! I went twice this year! It's possibly one of my favorite things to do. There are characters out you can only get at the parties, some of which are ONLY out at Halloween. Pretty much the only way to get Tarzan, Jane, Cruella, and Frollo outside of a miracle happening. Uncommon characters like Jafar, Queen of Hearts, Snow Queen, Abu are easy to get. A fantastic picture opportunity to get Snow White and all 7 dwarfs together that only comes at Halloween thought you will need to wait for that one a long time if you want it.
Trick or Treating all night long. TONS of candy and it's actually pretty good stuff. If you go down the trails in the last hour or so, they will give you like giant handfuls of stuff cus they are trying to get rid of it.
The "Boo to You" parade is one of my favorite parades. Only parade with Captain Jack in it as well as section based on the Haunted Mansion with the hitchhiking ghosts, grave diggers, waltzers, and the tune it super catchy. And the headless horsemen rides before the parades (weather permitting).
I don't bother myself with rides at the parties, but none of the lines seemed long. Only things I did were Magic Carpets which had no line at all and Haunted Mansion twice. Both times we walked right into the first room. ESPECIALLY after the second parade, the park was like a ghost town. We went to the last Villains Mix and Mingle and they were SO few people, some of the villains actually just came out and walked around in front of the castle. My friends and I hung out with Frollo for like 10 minutes. We had a blast.