We went to MNSSHP last year late Sept. It rained hard on and off for the first couple of hours and then light rain on and off the rest of the night. It was kinda miserable at first, because of the rain everyone ran to the ride they wanted, so the lines were longer than you would expect for a ticketed event. After the rain stopped all the lines got super short. Most people weren't wearing costumes or it was under rain gear.
The special character lines were way too long for us to get into with our 4 year old, so we skipped them. So we mostly rode rides, which after like 9pm were pretty short everywhere. Except 7DMT, that was still fairly long and broke down right before we were going to get on so they gave us a fast pass for anytime in the next couple of days.
We didn't do the trick or treating, but the parade, the sanderson sisters show and fireworks were AMAZING. We also didn't get to ride the haunted mansion that night, we ran out of time. Our 4 year old was completely passed out in the stroller while we watched the last show at midnight.
All in all, I think we would do it again, but not until our child is older. Also the rain kinda took the fun out of it, ruining the whole costumes part. We are planning on skipping it this year.