The New York Times has a good overview of when to prepay on a mortgage and when not to.
Basically if you've got a good mortgage rate, like we do at 4.78% fixed, the NYT advises that you should not be throwing extra cash at your mortgage if: (1) you have other higher interest debt (i.e. credit cards); (2) you are not investing enough in your 401k to get the employer match*; (3) you don't already have a good size emergency fund saved up.
I agree, generally good advice, our only debt is mortgage debt (although we've got more than one mortgage with our investment properties), we are maxing out our 401ks and we have a $30,000 emergency fund. So we have got the green light, according to this advice, to pay extra on our mortgage.
The NYT article also runs through the tax benefits of a mortgage (although I'm not in fan of debt for tax benefits), better returns elsewhere (i.e. investments) and the general need most people have for liquidity these days.
I'm always a little skeptical of not paying off debt to get a better return in the market, but I would assume that an individual who is disciplined enough to pay down a mortgage is also disciplined enough to invest or save that money rather than spending it. I also agree that msot people these days are in need of greater liquidity due to the general economic uncertainty facing many.
*I always find it interesting that almost all personal finance articles, advice, blogs, etc. assume that all employers provide a 401k match. I've never worked for a company that provides a match. I also don't agree that one should only invest in a 401k if one receives a match, invest in a 401k so you can take care of yourself in retirement.