We did MNSSHP on Halloween night this year. It was a lot of fun but I think it would be even better on a night when it wasn't so crowded. We saw Cinderella's carriage, the villains, etc but, since we'd paid so much extra for the tickets, we decided not to waste our time in long lines. There was plenty to keep us busy. Because even a sold out MNSSHP is no where near as busy as a 'regular' park day, you can ride anything with no line (literally-Haunted Mansion on Halloween night was no line!)
You get a TON of candy. We actually still have a ziploc bag full...in February. LOL To answer Soscary, even on Halloween the lines weren't that long, they do move fast. We found if one was long, we'd just try another one. The trick-or-treat trail was the best spot for trick-or-treating and for character greeting. The CMs give you a handful of candy at each stop. The CMs at the gates, when you leave, were giving handfuls of Ghirardelli candied almond sqares (yum!) Hallowishes are the best fireworks I have ever seen anywhere, period. They're like the Epcot Illuminations meet MK Wishes but with a Halloween theme.
You have to dress up because you won't have the same experience if you don't. Our family had very unique Disney-themed costumes and we ended up getting a ton of attention (and extra candy) from the CMs because of it. We even got a standing O from the ladies at the booths at the TTC!
I would definitely say to do it!!!
If anyone wants any more info or pics, feel free to PM me.