A credit card that helps pay down student debt?

Pencils with ABC blocks

Sallie Mae, the giant student lender, says you can use the rewards you accumulate from its cash-back credit card to pay down eligible student loans.

But if I were a student -- or a recent graduate -- I'd hesitate to sign up for this card, despite its sign-up bonus and benefits.

The Sallie Mae Cash Back Visa (www.salliemae.com) comes with a number of enticing perks, including:

-- a $100 cash bonus when you spend $500 within your first 90 days as a cardholder.
-- 5% cash back on gas, groceries and books.
-- 1% back on everything else.
-- 0% APR for the first 12 months.
-- No annual fee.
-- The ability to use cash back to pay down the principal on your student loans.

There are some limitations on the amount of bonus cash back you can earn, but they’re fairly generous.

For gas and groceries, you’ll earn 5% on the first $500 total combined net monthly purchases in those two categories. For books, you'll earn 5% on the first $1,000. Once you exceed those amounts, you’ll earn 1% back.

This is all very enticing, but students on a limited income could really get into financial trouble if they don't watch their spending or focus too much on accumulating rewards.

Ask yourself: Would I have the discipline to avoid spending beyond my means so I could pay off my balance in full before the regular interest rate kicked in?

After 12 months, the standard interest rate will be between 11.99% APR and 15.99% APR, based on creditworthiness.

And if the card’s rewards encourage you to spend more, you’re not going to come out ahead.

Keep making purchases with cash, and you might find you have more money available to put toward your student loan debt -- more than the 5% back you’d get with the card.

For responsible consumers who manage credit wisely and pay their balances in full and on time every month, the Sallie Mae Cash Back Visa is a great card.

It could net you $300 or more in cash back per year that can be used to shorten your loan term and total interest owed. The cash back cannot be substituted for your regular monthly payments.

Since the impact of getting cash back is immediate, while you won’t feel the effects of making extra student loan principal payments for years or even decades, it might be tempting to take the cash back.

But you’ll get more bang for your buck by putting the cash back toward your student loans (and not toward, say, a vacation) because you won’t pay interest on the borrowed money you repay early.

This card is best if you're already out of school, are financially responsible and are looking for a way to pay down student loan debt faster.

If that's your situation, the rewards on this card are competitive when compared with other credit cards.

The Chase Freedom card, for example, offers 5% back on gas and groceries, but only during certain months of the year. The rest of the year, cardholders earn just 1% back on these purchases. You also have to remember to sign up each quarter to "activate" the higher cash-back percentage.

Another contender for top cash-back rewards on gas and groceries is the Blue Cash Everyday card from American Express, which offers a year-round 3% back on groceries and 2% back on gas with no annual fee.

To get better rewards -- 6% back on groceries and 3% back on gas -- you’ll have to pony up $75 a year for the annual fee on the AmEx Blue Cash Preferred card.

If you're looking for a card to take to college, follow our advice on choosing a low-cost student credit card.