hi!we'll be in hongkong for 3days.its actually my 2nd time but i haven't used the octopus card yet so i'm wondering if it's worthy to purchase the octopus card with 2 single journey airport express and unlimited mtr ride for 3 days?eta is 9am and our return flight after 2 nights is 7.30pm.also,can it be used for fast food chains such as kfc and mc donald's?can it also be used in star ferry for hk-macau roundtrip rides?thanks!