Seems like an interest only loan is the smartest thing to do if you don't plan on living in your current home for more than 7 years. Considering most of your payment for the first 7 years is predominantly interest. Also, most of the money you typically make on your home is due to appreciation, not because you paid down the principal. An interest only loan makes even more sense for investment properties.

Right now you can get private 10 year interest only loans for 4%.

Another thought I had was if I was over the age of 70 and still had a fixed mortgage, I'd switch over to an interest only. The chances of dying are high so why pay for principal you won't ever see. An extra couple hundred dollars a month can really change the lifestyle of somebody on a fixed income.

btw I'm not a loan officer.