My 10th reunion was in 1999, so not all that long ago. There's a firm in Dallas that organizes these things professionally. They dug up all the names, contacting former parents or whatnot, and I believe asked us to pay some small amount of money to cover the event. No big deal, really.
There were actually two events: a Friday night "casual" get-together at a bar and a Saturday night "formal" thing at the top of one of the big office buildings. I think I preferred the casual event, but then I decided to dress up in my shiny new pinstripe zoot suit for the formal event. Half of the people thought it was cool and the other half typically said something like "so, what's with the zoot suit?" I was then voted most likely to become a politician.
I actually enjoyed catching up with some of my old classmates who I didn't necessarily consider to be friends back in the day. It was wild to see how some people became beautiful and others became ... less so. Several of my classmates about whom I was most curious never showed up. Still, I'm glad I went, and I think everybody should go if given the chance.