Demos' public policy expert Tamara Draut says credit card usage in the U.S. has dramatically changed in the last several years. "People used to use credit cards to stretch their finances for vacations, a new outfit or another type of luxury. Today, credit cards have become a Band-Aid to hold the family budget [together], a socially acceptable solution for maintaining living standards during periods of income loss or stagnation."

Just when credit goes from being a convenience to a problem, is hard to gauge. But there are warning signs.

Jim Tehan says, "Using credit cards to meet daily expenses is a huge warning sign that you have a problem with credit." He advises consumers who are wondering whether they themselves are running into trouble with credit card debt: "If you think you might have a debt problem, you probably have a debt problem."

Others say simply carrying any revolving debt is antithetical to a sound financial foundation. "When someone is carrying any credit card debt, it means they are falling behind instead of getting ahead, and that means there's a problem," says Harvard's Elizabeth Warren. "When people ask how much credit card debt is okay, it's a little like asking how much TNT can you keep in your basement. You probably could keep some and get along okay. But the smartest move is not to keep any."

The bar on where the red flag goes up for consumers is similar for the Consumer Federation of America. They recommend that consumers keep only one or two credit cards and avoid carrying balances unless it's absolutely necessary. "And people shouldn't even think about approaching their credit limit unless they're in an extraordinary situation," says CFA's Stephen Brobeck.

"Credit cards are the best of products and the worst of products," he says. "If you treat a credit card as a charge card and use it frequently it will be profitable for the banks and it will be free for you, and you may even get rebates."