My class had a 5-year reunion, but it was very poorly attended (about 25 people out of 400.) I didn't go to that one, I thought it was pointless since not much happens in 5 years.
I did go to my 10-year reunion, and I had fun. But the odd thing was that people still stuck with their old groups/cliques when they were chatting...old habits die hard? When we sat down to have dinner, most people sat with their old HS groups. (My HS had a reputation of being very clique-y, and I guess that's still true.)
I did end up talking to some people who I hadn't talked to since elementary school, which was interesting.
If we do have a 15-year reunion, I will proably go. But we'll see.