It's not the first thing on my "I'd like to do this today" list, but I'm fairly neutral about it. We always stay. Sometimes it's kind of fun to chat with the other parents, etc.
If DS isn't invited, DH usually goes and I stay home with DS.
Not sure what we would do with a party at McDonald's, though. We're vegans and can't eat anything on their menu (except ketchup, maybe

). It would probably depend on how well we knew the person throwing it. If it was just a random classmate that DSD wasn't that close to, we might be "busy." But do to all of the other options for kids' parties these days, I don't think that McD's is nearly the party destination that it was when I was a kid.