Hi - My sons and I attended
MNSSHP last year on Halloween! We all dressed up - DS1 as Jack Skellington, DS2 as the Mayor and myself as Oogie Boogie. The lockers were free that evening and that was awesome because we were super hot after awhile - we took off the costumes and stashed them in the lockers.
Anyway, there was lots going on. Even though we found the trick or treat lines to be long, (and sometimes hard to find)they moved quickly and we got a lot of candy. What we found to be terrific was the lines for many of the rides were short!!! We were on Splash Mountain 3 times when fireworks were going on - what a memory.
I think if you have little ones, the experience will be different because they will want to trick or treat more and see the characters or hang in Fantasyland where the ride lines were longer.
The costumes were a blast! The attendees were for the most part in good spirits. What we did do is arrive a full hour ahead of time in our pre-determined parade watching spot, for the 2nd Boo To You Parade. We saw the Headless Horseman and the Grave Diggers. The Villians float was great.

However, we bought snacks to eat while waiting and just people watching was fun!!!!
We had a great time-were some of the last to leave and although it did cost more, was truly worth it.
Would I do it every year - probably not, but I would do it again in a few years.
Just to note that family pictures are not included with tickets any longer.
Have fun!!