My 20 year high school reunion was held this past weekend. Though unfortunately I didn't learn of the reunion until just last week, I was able to attend one of the three weekend events.
I went to the Friday night icebreaker at a local restaurant-bar, and I'm glad I went. It was great seeing so many faces and talking to people from my formative years. What's more, most of my former classmates looked great, appeared to be in good health, and were very friendly. I talked not only with people I knew well in high school, but also some people I knew of. It was that kind of night. People were happy and really open to the event.
I did attend my 10 year reunion as well, but that seems like a lifetime ago, as so much has changed in my life since then (wife, house, baby).
Anyway, if anyone is oscillating about attending an upcoming reunion, I would encourage that you go. Of course, in my case, I neither loved nor hated high school and I had only a few close friends (although I was friendly at an acquaintance level with many other people), so I recognize that it was a very low risk, high reward proposition for me. Others with deep emotional scars from high school might not want to dredge up those memories.