I think the GO phone is great for teens. Put a little $$$ on the phone and when the money's out, they can't use it until they reload some cash, no overage charges! Good for learning the value of money, and you can buy the reload certificates almost anywhere, or just add on your phone with credit card.
Good: Good service with Cingular, largest calling area, few dropped calls. I've had cingular for >5 years with no problems. Price is good overall. I got one for DS13 for Christmas, bought him the basic phone and put about $25 every 3 months on (he has to pay for most of it). After every call, a text message appears telling you how much the call cost, and how much $$$ is remaining on the account. When you get below $4-5, you get a voice prompt before every outgoing call to let you know your balance is low. My husband and I both have cingular, so calls between the family are free. The above poster is correct, there are 2 plans available. Either 25 cents/min. or $1.00 flat fee for every day you use the phone, plus 10 cents/min (except calls to other cingular phones are free). When i transferred my plan to a GO plan, I just switched my number and paid a $25 transfer fee. I didn't have to purchase a new phone.
Bad: All incoming calls go by the same rules, so wrong numbers (even hang-ups) cost too. If her friends call her alot, she'll run thru the cash very quickly.
I would try to calculate how many minutes/month she uses (or would use) and see how much it would cost monthly. I actually switched my regular plan to the GO phone because I had less than 200 minutes monthly, mostly to DH cell phone and didn't use my phone every day. My original cell phone cost $49/month, now I spend about $20. You would save more money if both you and DH switched to Cingular as well. If you did that, you could consider the family plan. Purchase a package with a huge amount of minutes, then I think adding a family member is $9.99/monthly. I would also look into Disney mobile. Can't beat GPS tracking on your kids, especially a teen! Don't know what the prices are, but I would think this newly launched service might have some good deals.