We were in WDW this past Halloween week and had an absolutely amazing time. We did two MNSSHPs - the first on the day we arrived (Sunday, October 26th) and then the one on Thursday, October 30 (which sold out). Both were a lot of fun, but we definitely got more rides and things done on Sunday night. By the time Thursday came, we pretty much enjoyed watching other people and taking our time to take in the atmosphere. It wasn't unbearable, but compared to the Sunday party, it was packed.
In general, the park crowds were terrific!! We got everything done that we wanted to, without going crazy or waiting in lines that were really long. Between Fast Pass and smart park strategy, we never waited more than, I'd say, 15 minutes. It actually became a rule for us! Especially at
MNSSHP, we were able to ride the E-tickets several times. We did Space Mountain more times this trip - without waiting in line - than probably all of our other trips combined....and that's a lot of trips!!!
Incidentally, we also did two days at Universal during the week, and found it to be even less crowded than Disney. We could've easily done all the attractions in both Universal Studios and IOA in one day.
I'd definitely visit WDW during that week again. If I had to choose between MNSSHP dates, I'd probably suggest the earlier the better, but if you don't have that option, going on a sold out night is worth it too.
If you have any other questions about the week or just want more info, please don't hesitate to ask!!