When it comes to future payoff plans, "everyone has the best of intentions," says Wu.

Bowne agrees. "What we've seen over the past few years is that people have every intention and ability to pay off the loan," she says. Then "unexpected things occur, and suddenly they can't make that payment every month."

So weigh the odds. Do you have the money to pay off the purchase in cash now? Can you set it aside, just in case? Is the purchase a want or need? Do you have other payment options?

In some instances, you may even be better off using a low-interest credit card over a deferred-interest plan, says Frank. "There's a higher chance you'll end up paying interest than you think."

Always ask about alternatives, says Wu.

"You don't want to use it for hospital bills," she says. Interest-free medical financing can carry high interest rates when the deferred-interest period expires, and many hospitals offer more reasonable payment plans and other options. And in a few states, alternate payment options are required for eligible patients, she says.