I had my first experience on one about a month ago. My Domme and her sub/junior Domme (Liza) took me (their "novice sub") to a community event back in June. It was at a house out in the country with a dungeon with several rooms in the basement. There were maybe 20-25 people at the party - Dommes & Doms with single or multiple subs.
Anyway, upon arrival I was immediately told to strip and and was escorted to one of the dungeon rooms that had a medical exam table, a St Andrew's cross (with leather padding), and several chairs and a folding table with stuff on it covered with a sheet. There were 4 subs in the room, two women and two men (including me) - all of us naked. After meeting the dungeon master and discussing safety protocols, the dungeon master directed the two female subs to fasten me to the cross. My ankles and wrists were strapped in accordingly. Then the cross was tilted back (maybe 15-20 degrees) and a blindfold was put over my eyes. I heard, but did not see, someone direct one of the other subs to to put on a hospital gown and sit on the exam table. The other two were told to stand against the wall and await commands.
After a bit, a voice said "is this one your's?" And I heard my Domme reply. They spoke softly and then I heard a female voice purr in my ear "Comfy" and when I started to reply both of my nipples were twisted hard and I was told to "STFU!" and the evening progressed from there.
I remained blindfolded the entire time. I was ignored, fondled, licked, slapped, tickled, hit with what felt like a riding crop, verbally praised and subsequently degraded (all stuff i had agreed to previously - although not in detail - no one ever encroached on my "no-go" areas/activities).... At one point I was fondled & stimulated to erection only to then be doused with ice water. There was liberal use of e-stim, ice, and heat (and it was excruciatingly wonderful).
At one point, I listened while the medical exam took place on one of the female subs in front of an audience - it sounded humiliating, a little painful, and alternating wonderful - judging from the intensity of her orgasms. During the exam, one or more people would fondle me or stroke me, and then leave me alone.
After the exam was done it was time for orgasm denial with me - with plenty of humiliation, derogatory statements, and suitable punishments. I never knew the size or make up of the audience - and that was one of the things my Domme knew that I wanted.
Overall, I thought it was a great experience. I was sore and bruised the next day (as I expected). My Domme & junior Domme were both pleased and rewarded me appropriately.
Everyone is different, so make sure you discuss limits & desires before getting strapped in - and make sure that there is someone you trust completely present.