I would probably fit your "less popular" category, as most of my friends in high school were from neighboring towns and the place I worked. I went to my 20th, as I heard that was the one that the cliques disappear and people tend to be friendlier. That didn't turn out to be true in my case.
It was an interesting experience to see how people turned out and I did catch up with a few friends, but, overall, I would say the whole experience was "meh".
Probably worth doing once but won't go to any more (my 25th is this summer).