What you want to be sure of, however, is that you are investing the extra with the same diligence and commitment as you would if you were sending the extra to pay down your mortgage. If you don't, and just end up spending the extra, then it won't work and you would have been better paying off your mortgage early. What I have found, however, is that followers of Dave Ramsey have incredible diligence, but because he doesn't actually explain how money works but rather just instills a fear of debt, they use that diligence to make financial decisions that are not as wise as they otherwise could have made.
Ramsey also gives bad investment advice, so while he might inspire you to get out of debt (which is a really good thing), you need to learn from someone else what to do with money when you have it. And this is a good example of where the two cross over and not understanding will set you back. On my mortgage, I'll have made $20,000 in dividends from the leverage between the time I took it out and the time I'll pay it off. I'd like an extra $20k heading into retirement that I didn't need to do anything for.