Cratered interest rates, a still-scary stock market and the eroding real-estate situation make it vividly clear: The only decent, guaranteed return you can find these days is by paying off debt you've already incurred.

If you're carrying credit card debt at 11%, for example, every dollar you pay off earns you an instant 11% return on your money. That's a great return in a world where:

• One-year certificates of deposit average around 2%.

• Yields on short-term Treasury securities are close to zero.

• Many retirement funds are down 40% or more from their peaks.

• Home prices are still in free fall.

So shouldn't we be tackling all our debt, including our mortgages?

Not so fast.

There are situations where paying down a mortgage makes sense, such as when you're approaching retirement or when reducing your principal will get you a much better deal on a mortgage refinance.

But most people still have better things to do with their money, even in this environment, than to pay down a low-rate debt that's often tax-deductible to boot.

And if you're "underwater" on your mortgage, paying it down is probably the last thing you should do.

The urge to be free of debt

It's not that I don't understand the impulse to speed up the day that you own your home free and clear. There's something psychologically satisfying about knowing the bank can't take your castle.

Besides, the numbers can seem pretty impressive. Let's say you have a 30-year, $250,000 mortgage at 6% interest. Your monthly payments are $1,498.88. By paying an extra:

• $100 a month, you could save nearly $52,000 in future interest and pay off the loan four and a half years early.

• $250 a month, you could save nearly $100,000 in future interest and pay off the loan nine years early.

• $500 a month, you could save nearly $144,000 in future interest and pay off the loan almost 14 years early.