We went as a group of seven adults last year. The night we got MNSSHP tickets for was quite possibly the worst night of the trip. There was a big rain storm about an hour and a half before the event started. It was part of a larger storm that was looming on the horizon. We never got the thunder and lightning portion of it, but you could see it off in the distance.
As a precaution, they shut everything down, even the train. Luckily, that was the time we had our dinner scheduled for. So we got out of the downpour (not before getting soaked) long enough to eat. When we came back out, the rain was pretty much done. However, they delayed the parade and fireworks for over an hour as a result. The fireworks were absolutely amazing. A lot better than the standard show, and I love the standard show. The parade was also amazing, but unfortunately we didn't get the headless horseman.
It was a terrible situation, and there was a huge line of people attempting (to no avail) to get their tickets refunded at Guest Services. But we stuck it out, and enjoyed every second of the somewhat shortened experience. Once the Fireworks and Parade finished, we got maybe an hour to ride rides before the park closed. And most of that time was impossible to move around due to the second parade coming through. We also didn't have the person interacting with guests at The Haunted Mansion.
Moral of the story, MNNSHP is incredible. But pick your night wisely. We got our tickets in advance. If I had to do it over again, I would've waited until we were close enough to see the weather for the week before getting the tickets. I'm 31 now (30 at the time), and while I went with a large group, we were all adults who had no shame in loving every second of the event.
The Group