I had my tongue pierced years ago. Was painful for all of about 10 seconds, and then just uncomfortable , but nothing I couldn't handle.
Best piece of advise I can give, is if you do have it done, and you are told to rinse your mouth with listerine or other mouthwash, always dilute the mouthwash with water. I religiously rinsed my mouth with listerine as instructed, and within a week, had a white tongue. I thought I'd contracted something horrendous, and that my tongue was practically dying (or something equally as melodramatic), and I was told that the listerine had killed all the healthy bacteria, meaning that the normal tongue cells weren't removed, causing the whiteing effect. (After laying off the listering the piercing healed perfectly and my tongue looked as good as new, but with a new accessory ;)
If you do have it done, definitely go to a well known piercing place - main risks are having a blood vessel pierced through, or it affecting the nerves etc in that area - a professional piercer will examine your tongue and tell you whether you have an increased risk of anything like that (ie if your tongue's blood vessels are very close together etc and too risky). I've also heard that it can slightly increase your chances of having a stroke in later life, but there are always these types of risks etc.. and as far as I'm aware they are very very rare.
:)