Here is a story of a girl having piercing done:

“I got it done when I was 14 (with an altered school id) and I am now 22... and it was sorta outdated even then... but I didn’t care. The other thing is it totally screws up your teeth and your teeth are importanntttttt. I ended up taking mine out when I was about 17... and my mom was nice enough to pay for me to have some cosmetic stuff done on my teeth (expensive)... so no more tongue rings for me. I hate the way it looks when teeth are chipped... and trust me... they'll get chipped even with a plastic barbell. Its not worth it... and they’re not even cool anymore (when I see girls with them I think they/re gross looking... like go back to the 90s and put your tongue back in your mouth stop showing off) I hate girls who stick out their tongues JUST to show off their tongue rings... although I did it too, but I have an excuse because I was a dumb teenaged girl... trust me... you’ll get sick of it fast.”

Tongue piercing can eventually destroy front and back teeth from the tongue bar continuously hitting or rubbing up against teeth. The bars eventually wear away the enamel on teeth, creating additional problems associated with teeth, including massive pain. Once the enamel is gone, the dentinal tubules (microscopic "holes" in the teeth) are exposed, which creates massive pain, on the same order of a massive toothache when a cavity is there. This in turn, will create the need for further dental care, which could include anything from crowns (caps) to partial dentures to implants.

The only pro, besides them looking nice, is that the mouth is one of the fastest healing parts of the body.

Here is some cool advice:

According to the American Dental Association (ADA), tongue piercing typically causes:

    pain

    inflammation

    infection

    greater saliva production

    chipped or cracked teeth

Less common side effects include:

    injury or shrinkage of gums

    damage to teeth, fillings, and other dental work

    difficulty chewing or swallowing

    prolonged blood loss immediately after procedure

    possibility of disease transmission

Modified from a post and full credit given to alphonsians93