I've talked to a lot of people trying to pay off their mortgage before retirement.  It really could be the ultimate "peace of mind" and a great goal for a comfortable retirement.  This article from the Christian Science Monitor explains some of the math.

Here's how the math works in the case of a couple with a 25 percent federal tax rate and a 6 percent rate on their mortgage. Because mortgage interest is tax-deductible, their effective interest rate, with the tax-benefit figured in, is 4.5 percent. If stocks return about 7 percent a year - which some analysts see as a reasonable forecast - the couple stands to gain by investing rather than paying down the mortgage. But much of the financial benefit vanishes if the investment isn't tax-sheltered.