I've opted out over 50 times (and I try to help collect data about screening methods on FlyerTalk, though I keep thinking this needs to be organized better -- there are so many people collecting it in separate places, including different FlyerTalk threads and different web sites). I think it's interesting that the attitudes of screeners, as well as the thoroughness of their pat-downs, has varied so much. (Some of them have tried to talk me out of opting out, others have been very matter-of-fact, others have fairly vocally criticized the body scanners or TSA itself.)
Three times there was another person opting out within my sight and at least twice I got to talk to a fellow opt-outer (opter-out?). As I recall, their concerns were always more health-related than privacy-related, unlike mine, so I had a bit less common ground with them than I first expected to.
On two consecutive flights (more than a week apart and from different airports) I alarmed the ETD and got detained and subjected to multiple pat-downs and a hand-search of my luggage. I still haven't figured out what caused it, but I put my shoes and backpack through the laundry and it's never happened again. One hypothesis is that I might have gotten a drop of plant food on my shoe, which has made me much more paranoid about touching fertilizers.