I had my tongue pierced for awhile and I cringe every time I remember the "clank" and "thud" of my barbell against my teeth. I've also heard that the majority of piercings migrate and that sounds painful and would nullify the process in the first place.
Now, nipple piercings...I'm thinking about asking for it as a birthday present.
I love the look of the really thin delicate hoops and I don't plan on having children so the consequences would be minimal. I'm barely into breast play as it is, if they become less sensitive it would be a huge deal.