I know someone with one. Not me, before you ask, as I’m not into piercing. 🙂
Apparently it is fairly painless to do and you’d be fitted with your first ring. You have to be careful with this as it will be fairly thin. You’ll be encouraged to clean it for a week or so then play with it, in order to stretch the hole. After a while you go back and get a larger one fitted. The weight eventually stretches the hole for you. You continue stretching/replacing until you reach the width of ring you want.
You have to be careful with the first ring as it will be thin. Wear briefs or combines (everything safely gathered in!) as you don’t want the ring to act as a cheesewire. 😯
The benefits I hear are two-fold. More sensation for you and more sensation for him/her. You’ll have to refrain from sex for a while after piercing to avoid infection. One drawback may be being careful with teeth and breakages thereof 😛
And it *doesn’t* set off airport metal detectors if you’re worried about that.