I have mine done, and I actually got it when I was 18 because EVERYONE else was getting their eyebrows pierced, which is what I originally wanted. (I'm babbling, sorry!) Anyways, while almost every pro and con mentioned it true, it is different for everyone.
First off, THOROUGHLY look around your local piercing/tattoo parlors for the one that is MOST sanitary and CERTIFIED! That's number one! I was stupid and went to a place that didn't look the cleanest, but I knew someone there that I trusted with my life. Needless to say, I still have it and I've had NO complications. Just because they're good with ink, does NOT mean that they know how to pierce CORRECTLY, so make sure you do your homework!! There are websites online where you can search for PROFESSIONAL PIERCERS in your area!
Your tongue DOES swell for the first few days. Here's a secret; once you get it done, go STRAIGHT to the nearest subway and get a cup of ice. MOST have the ice cubes that are cylinders, but they're hollow all the way through. Therefore, you can put your new barbell through the hole, and the ice helps with the swelling. (I also recommend taking some advil or motrin the first couple nights because IT WILL BE SORE in the mornings!!!) DEFINITELY gargle with water and sea salt, or listerine after EVERY meal or snack for the first week!
You're not supposed to change the barbell for 6-12 weeks. The time depends on what your piercer recommends. Anyways, DO NOT change it before then, it hurts! I didn't want to get caught with mine when I first got it done, so I put in a clear plastic one.
I never got a lisp except for the first few days where it was swollen. Mine didn't get NEARLY as bad as some other people I've seen with them. Also, while some peoples tongues close up right away, mine I can leave out for a day or two before it starts to close up.
RISKS:
You have a lisp for a few days, OR if your pierced too close to the tip of your tongue, you can have a lisp until it is removed.
**IF the vein in your tongue is hit "just right" it can paralyze you. HOWEVER, if your guy/girl is CERTIFIED AND EXPERIENCED, you will be fine!! While this is a scary thing to think about, this has not been the case for MANY years!!
Your dentist will scold you, but the first three years I had mine, they were amazed at how my teeth weren't chipping at all. It's the people who ALWAYS play with there's that end up having those chipping issues. I originally got mine to be "different," but then it became a fad everywhere.
It's a piercing... in my opinion, if you don't like it, you can take it out! It's not NEARLY as scary as a tattoo which will pretty much be permanent.
Sorry for writing a novel, but I'm not even sure I got out everything I wanted to say. Hopefully, it will help with your decision if you haven't already made up your mind!