Interest Only Mortgage

I’ve been hearing a lot of conflicting stories about interest only loans. Recently I was told if you could find out what your monthly mortgage would be if you you had a traditional 30-year mortgage ex. $1000. By getting the interest only loan could drop your payment to ex. $600 and if you apply the $400 savings directly to the principal EVERY month then you would build equity and pay off your mortgage in less than 15 years. Can anyone tell me if this is fact or fiction?

A FACT! TAKE A MORT. @ 100,000 ON A 30 YR TERM @ 6% RATE. NORMALLY IT WOUL BE 599.95 A MO, BUT I/O IT WOUL BE 500 A MO.