For people who are half decent with money the companies that offer "12 months no interest" type plans are the best. In theory, it allows you to invest the capital up front for the 12 months and then pay it off. You essentially can reduce the cost by whatever the prevailing interest rate on a 12 month CD is.

But yes, these plans (usually deferred interest) are designed to exploit people who are bad with money. They also usually go out of the way to hide how much you're racking up in order to maximize gain. This kind of person thinks "What's another $1000" when their bill is already at $4000. Furniture stores are infamous for this.