(MONEY Magazine) – THIS MONTH: --Line up a mortgage before you go house hunting. --Global CDs glitter, but the risks are high.

The lure of getting something for nothing is irresistible, which is why so many Americans shop with rebate cards--credit cards that let you earn points toward airline tickets, cash refunds or other goodies every time you charge. Such cards now account for roughly one out of every four credit cards in use in the U.S. But they are becoming victims of their own success. To offset the cost of the incentives, many card companies have begun reducing rewards and raising interest rates and fees on their rebate cards. So you need to take a hard look at whatever card you use to make sure it's still a good deal. (For a selection of cards with an attractive combination of credit terms and rebate offers, see the table below.) "A card that was once well worth its cost may no longer be worth what you're paying to use it," says Sara Campbell, senior vice president at Bank Rate Monitor, a consumer-banking research firm.

Consider these recent cutbacks: Last year, GM MasterCard sliced the maximum award that gold-card holders can earn toward the purchase of a new car by 50% to $500 a year or $3,500 over seven years, down from $1,000 annually and a total of $7,000. Sunoco MasterCard has cut the cashback rebate on nongas purchases from 2% to 1% (though it upped rebates on gas purchases from 3% to 4%). And all major airlines have now hiked to 25,000 from 20,000 the minimum points cardholders need to earn a free ticket.

Other card companies are making their offerings more expensive to use--or dropping them altogether. In January, the GE Rewards MasterCard, which has a cashback rebate of up to 2% on purchases, slapped a $25 annual fee on cardholders who don't carry a balance. In April, Pacific Bell Visa and MasterCard--which offer a 1% rebate on purchases that is then used to reduce your Pac Bell phone bill--upped the interest rate from prime plus 8.4 percentage points to prime plus 9.99, bringing the rate to 18.49%. And in June, Ford said it would stop offering a 5% rebate on its Visa and MasterCard purchases, payable toward a new Ford.

Moreover, experts expect the cutbacks to continue. So in light of recent changes--and those yet to come--answer these questions to be sure your rebate card still makes sense:

--What is the card costing me? If you carry a balance from month to month, the higher interest rates that rebate cards typically charge almost surely outweigh the benefits you receive. Example: Great Western's rebate Visa card, which has a 19.4% interest rate and no fee, offers a 3% cash rebate on all new purchases if you carry a balance. In contrast, Great Western's basic Visa has a 12.25% interest rate and an $18 annual fee. If you run up a $1,000 balance on the rebate card and make the minimum payment each month, you'll pay $184 in interest after a year and earn a $30 rebate, making your net cost $154. But if you carried the same balance on the standard card, your total cost would be only $130.

--How valuable is the rebate? Free airline miles and other rebates sound great but may not be worth as much as you think. Using a Citibank AAdvantage card ($50 annual fee; 17.9% annual interest rate), you would need to charge $25,000 within three years to earn a round-trip economy ticket within the continental U.S. on American Airlines. A recent 14-day advance-purchase fare for a flight from New York City to Los Angeles is $341. Thus you're earning just 1.4 [cents] for every dollar you charge. Look at it that way, and you may decide you'd be better off using a cheaper credit card and buying the ticket.

--Will I spend enough to collect the rebate? Even the best rebate deal is no deal at all if you never use the benefits. In general, credit-card issuers figure that only 50% of rebate-card customers ever collect rewards, according to banking industry consultant James Accomando. One reason: The average consumer racks up about $2,000 a year in credit-card charges. But you need to spend $25,000 within three years to earn most airline tickets, for example, and $10,000 a year to garner the maximum $500 benefit from a GM MasterCard. If you don't run up big charge bills, rebate cards may not be right for you.

The lesson: Don't let a rebate blind you to the costs of using the card. If you carry a balance, you want a card with the lowest interest rate you can find. If you pay your balance in full each month, you want a card with no annual fee, or one whose rebate is more valuable than the fee. Make sure you've got a decent card, and then any rebate you earn will be icing on the cake.