I think they are a good idea. You put the amount you want on the card and you can replenish it any time you desire. (At least that is how Visa Buxx works) And you get to check out where the money is spent! You can check to see the balance and everything.
I didn't use the card for a teen. I used it for my brother who was taking care of my mom the last four months before she died. I took care of the finances, but since he lived in another state he needed money for buying groceries, gas, sundries. I handled all the bills, etc., and the card made it easy to send him the money he needed. It worked out pretty well.
I think the cards are excellent and since you can monitor the expenditures, you can discuss with your child wise spending and unwise spending.