Not to seem immodest, but I skipped a grade and so my 40th was last year.
I didn't go.
First of all it would have meant spending a fair size of money on travelling back to New York and secondly there really wasn't anyone I wanted to see.
I'm sure that if I had gone I would have enjoyed reconnecting with some people with whom I actually never connected in HS, but it didn't seem worth it.
Around our 30th or 35th I spoke with someone who I considered a "best friend" in school. I found he had completely lost his sense of humor, which was the basis of our past friendship. It soured me on reunions.
My sister-in-law went to her 40th two years ago and sent me pictures. We went to the same HS and I hung around with a lot of kids in her class.
Seeing the stark difference between what they looked like now and what they looked like in my memory was disturbing.
I would rather think of them in their youth.
I have kept up with my closest friend from school all of these years (he was a year ahead of me; with my sister-in-law), until he blew off my daughter's wedding a a couple of years ago.
For most of my life I've found friendships to be almost sacred, but these day I find that Family trumps Friends.