I think it really depends on the person. I'm a "rare" sort that got "really" screwed up from dental work. I mean "really". I had Chronic Fatigue and full blown Multiple Chemical Sensitivity, along with full body rashes after eating, headaches that would make most people cry, cramps and pain in the gut for 3 hours after eating and a dozen other symptoms I won't get into.
I just posted the warning for people in really bad shape like me. You have to be careful. If you have Amalgams, but they aren't giving you a problem, then its probably not an issue to Zap.