(MONEY Magazine) – The letter G stands for giveback these days. Or so you might conclude, now that three of America's biggest corporations -- GM, GE and GTE -- have come out with MasterCards that offer discounts or rebates on selected goods and services. You'll need to study these G-cards bearing gifts carefully before you sign up, though. Hidden expenses make them worthwhile only for the minority of cardholders who can really take advantage of the discounts. The cards, launched separately during a 19-day span in September, are part of a trend sweeping the industry. Manufacturers and service companies sponsor cards with givebacks in order to promote themselves or boost their sales. Such deals have been around for nearly a decade, but they really took off in the late 1980s when American and United Airlines brought out versions that awarded frequent-flier miles based on how much you charged, regardless of whether it was for air travel (for a rundown, see Travel Wise, MONEY, April). Telephone companies soon followed, capped by AT&T's Universal card, which provides a 10% refund on long-distance calls you charge. Sears' Discover card, though it doesn't confer regular discounts on goods, gives you a cash refund equal to 1% of your annual purchases over $3,000. Sears said in September that it would spin off the card next year, but analysts say there's no chance the card will drop its cash-back feature. Americans now carry more than 18 million of these giveback cards. ''They will be the dominant wave of the '90s,'' predicts David Robertson, president of the Nilson Report, an industry newsletter. ''They give consumers a discount and companies a chance to put little plastic billboards in everyone's wallet.'' Each of the three new cards adds a novel twist (for details, see the table on page 27). GE's Rewards card, potentially the most generous one, offers rebates on GE products plus at least $800 a year in savings certificates good at national retailers like K Mart and Toys R Us. GM promises a credit equal to 5% of your charges -- up to $500 a year for seven years, for a maximum $3,500 credit -- toward the purchase of a GM car or truck. And GTE plans to give its 13 million customers in 40 states 10% off the cost of calling-card calls and products at GTE phone stores (the company has sent fliers to 70% of customers so far; the rest will get them next year). To decide whether these deals make sense for you, consider your spending habits. If you are among the 70% of cardholders who carry a balance from month to month, they can be costly: GE's card charges a variable interest rate equal | to the prime lending rate -- recently 6% -- plus a whopping 12.4 percentage points, whereas the average variable-rate card adds on 9.5 points, which is bad enough. (GE says some customers will pay only 8.9 points over prime, but it won't say which ones.) GM's rate is prime plus 10.4 points, and while GTE's card comes with an eye-catching initial rate of 6%, that teaser will jump to match GM's next May. With such high finance costs, you will need to make good use of the discounts to come out ahead. Consider GE: If you charged roughly $2,400 a year and carried a monthly balance of $1,644 -- both national averages -- you would pay about $302 a year in interest plus a $25 annual fee. That's a total of $122 more than you would spend with a low-rate card like that from Amalgamated Bank of Chicago (800-365-6464), which charges only 12.5% interest. So just to make up the difference, you would need to redeem at least $122 in savings certificates. And that may be far harder than it sounds; most of the credits expire in only 45 days, and some stores require you to buy goods worth $50 for each $10 discount. Given the same assumptions, the GM card would cost about $64 more than the Amalgamated card; to come out ahead, you'd have to get your $120 rebate by buying a GM vehicle. And GTE's card -- after its rate rises -- would also run about $64 a year more than Amalgamated's, meaning you must spend $640 a year on calling-card calls and phone gear to break even. Even if you pay your credit-card balance in full most of the time, giveback cards can nip you hard in those occasional months when you leave some of the balance unpaid. Do that with the GM card and you lose your so-called grace period -- in effect causing you to incur an extra month's interest that most cards forgive. ''It's a way of squeezing a few more dollars out of cardholders,'' observes Robert McKinley, president of RAM Research in Frederick, Md., which tracks credit-card rates. The GTE card offers heavy borrowers a rare opportunity to get the credit monkey off their backs, though. First, apply for the card. Once you're approved, you'll receive checks that you can use to pay off any existing debts by charging them to the new card at its low 6% rate. Then pay down the balance on your GTE card before next May's rate rise. Assuming you transfer $6,000 from a card that charges the national average of 18.45% interest and retire the debt in six monthly payments of roughly $1,000 each, you would save about $233 in interest costs. . CHART: NOT AVAILABLE CREDIT: NO CREDIT CAPTION: HOW THE THREE NEW GIVEBACK PROGRAMS MEASURE UP Using the cards described below is like eating a pomegranate: There's lots to chew on, but precious little juice. Still, for some that payoff can be sweet -- particularly from the GE deal, assuming you spend enough to reap discounts that offset its cost. All the cards have variable interest rates pegged to the prime lending rate.