FEHB will cover you somewhat during international travel; Medicare will not.
Most people I know who have retired from the federal government have both Medicare and FEHB together, though. FEHB works as the supplemental insurance to picks up much of what Medicare doesn't cover. If you have serious health care costs, Medicare+FEHB is a great deal, especially if you have a family. My husband was always incredibly healthy in his younger years, but developed an extremely serious illness in his mid 50s. We have thankfully paid very little for any of his medications, treatments, hospitalizations and therapies because of having his federal gov't health insurance as well as Medicare.