It doesn't build up any credit. Fees are high.
But, for people who can't get credit they are a good idea.
Fees.
What a joke. You pay someone to practice going into debt. So you can go into debt for real to "build" a high fico score so that you can get into more debt.
If you are responsible with your money and live on less than you make. If you pay as you go and save up an emergency fund. You will not have any payments. You will have money and you will not have to worship the great fico.
I don't see one. They are easy to obtain. If you follow the rules and use common sense you will build your credit. After you are established, you already have the cash available to pay it off.
Keep your balance low to avoid any interest though. Be sure there aren't too many fees as well.
Most of them are, in my opinion, ripoffs. This is because they have so many fees it is ridiculous. Fees for activation, balance checking, each transaction, refilling. Also, if you want to dispute a charge, it is difficult to do so. Cash is better.