It is important to understand the difference between Traditional Medicare and Medicare Advantage Plans.
1) As misleadingly advertised on TV, Medicare Advantage plans are cheap with all kinds of benefits, but one has to stay in their network, get permission for appointments and tests and pay copay. Certain Mayo Clinics have no agreements with Advantage Plans and lots of other health providers drop them as well - in Minnesota recently 13,000 Minnesotans were told that next year their Humana Medicare Advantage Plan would no longer be accepted at Health Partners clinics and hospitals (a very big organization). They must go elsewhere or get a new plan - and while all plans need to take you when you first go on Medicare, later when you want to change, they are no longer required to take you and if they do take you, they can charge a lot.

2) Traditional Medicare allows one to self-refer and go to any doctor that accepts Medicare in the United States. Add a quality Medigap policy and you will never see a bill. That costs more but when you begin to have problems you are very glad!