(MONEY Magazine) – More and more credit-card reward programs are tied to the amount you charge. But not Mellon Bank's new plan. The Pennsylvania bank promises to refund every penny in interest you pay on carryover balances if -- and it's a big if -- you hold the bank's new CornerStone MasterCard (17.9% annual interest rate for the standard card, 14.9% for the gold; no annual fee) for 20 years. You can get back 10% of your interest after two years, and the proportion increases from then on, but once you take it, the waiting period begins again. A good deal? Well, figuring that out requires so many assumptions about future interest rates, market conditions and taxes that the exercise is essentially meaningless for most people, says finance professor Gerald Hanweck of George Mason University, who analyzed the card for MONEY. Furthermore, you're always better off paying your balances in full each month. If you're unable to do that, apply for cards with low interest rates, such as the ones listed on page 35. Then take the money you save on interest (compared with higher-priced cards) and invest it or, better yet, use it to reduce your balance. Mellon's plan is eye-catching. Who wouldn't mind getting, say, an $8,667 check after 20 years (the amount Mellon says it will return to the average customer who carries a monthly balance of $2,421) -- even if the money will have been cheapened by inflation? But there are smarter ways to play your cards.