I've done this in a group of adults--two 20-somethings and two 40ish. We had a blast!! Two of our group really hated super scary stuff, so HHN at Universal was out. MNSSHP is just fun. Lots of character meets, the obligatory free photo, crowds aren't too bad, and it's one of the few opportunities for adults to dress up in costume and go trick-or-treating. What could be better than that?? I hope to do it again this year... although I also want to head over to Universal one night as well.
DH and I went in 2003 also and are going again this year. We liked MNSSHP better than MVMCP but that could be because the weather was nicer in October.