Read all policy descriptions and exclusions carefully, often the ones you buy on the airline websites don't have extensive coverage. Typically, with any insurance, the medical condition has to be severe enough to prevent you from traveling. Not just a doctor's note, they send full detailed paperwork to be filled out by the doctor and the hospital.
That said, if you are booking with Disney Travel Company, or a TA that will use Disney Travel Company, the Disney insurance is a flat rate of $77.50 per adult, under age 18 are covered for free, and this ins. WILL cover any flights that you book yourself for your WDW trip, unlike many other travel suppliers whose ins. will only cover what is purchased through them only.
Disney uses Travelex, who I find is very fair in covering claims, but you still need all the relevant paperwork and claim forms.