That sounds good, in theory. But in reality, many consumers are unable to take advantage of these benefits because they carry a balance on their credit card from month to month, paying finance charges that can go up to a whopping 23 percent. Many find it hard to resist using the old "plastic" for impulse purchases or buying things they really can't afford. The numbers are striking: In 1999, American consumers charged about $1.2 trillion on their general-purpose credit cards.