If your current mortgage interest rate is five percent, you are guaranteed to «earn» five percent — by saving interest — on any amount of principal you pay off. (themortgagereports.com)
A few hundred dollars in extra principal paid in the first few years of a 30 year mortgage can remove years of interest payments from the mortgage term. (money.stackexchange.com)
Since you're getting principal paid back each month, you could turn around and invest it quicker - much better than keeping your principal tied up in a certificate of deposit. (thecollegeinvestor.com)