From a long-time C-ring lover, I'd say rubber ones are better than metal ones, because they have more "give" in them. And don't wear one all day, unless it is just big enough that no matter how you're standing, sitting, or lying down, that you won't cut off any circulation. The general rule of them is: don't wear one with an erection for more than about 20 minutes. That's good advice. Just for one example, I fell asleep one night with one on, and you probably know how many separate erections the typical man has during the typical night of sleep. Bad idea, although I guess no permanent damage.