Swelling numbers of cards means more choices than ever. And that applies to fees, too. Depending on the card they choose, consumers can be hit with many fees that rapidly add up.

Most prepaid debit card fees are predictable, such as those for activation or reloading. But there can be unexpected fees too -- calling customer service, checking balances and even inactivity -- that can add up over time. That's why it pays to do some extra digging, says Michelle Jun, a staff attorney at Consumers Union.

"Fee structures aren't as simple as consumers expect," says Jun. "So they have to be aware." Between activation and initial load fees, consumers can pay up to $30 just to buy a prepaid debit card, according to Consumers Union.

Fee-hunting aside, some consumers can benefit mightily from the cards. They work much like regular debit cards, because they're reloadable and can be used to make payments or purchases. Spending is limited to the balance on your card.

So, the prepaid debit cards are well-suited to people who are locked out of traditional bank accounts, travelers, and parents who dish out allowances to their kids. People can also use the cards to shop online or pay bills. Spending is easily monitored because you're not paying cash. And most prepaid debit cards don't charge fees for card purchases, according to Consumers Union.

"The products are evolving in sophistication," says Elizabeth Robertson, director of payments research at Javelin Research & Strategy. "There's a broader array of alternatives." The research firm estimates that prepaid card usage will more than double by 2014.

Because the cards are typically co-branded with a major credit card company, she adds, you're probably protected from fraud. "The liability policies of credit card companies apply," Robertson says, "so that offers extra protection."