My husband and I were in Hong Kong last September and bought an Octopus card before even leaving the Airport. Second (third?) everyone else's comments about the convenience of the card. Our last day in Hong Kong, we had run out of money on our cards and had to resort to paying the exact fare in change for the Star Ferry, etc. What a pain. (You can recharge your card with extra money, but there is a minimum recharge value that was going to exceed what we would be doing that day.)

FYI: The Octopus card can also be used at many (most?) convenience stores; again, very convenient to just swipe the card when purchasing a couple of bottles of water, rather than digging in your pockets for change. Also, I think that each person must have their own card; you cannot use your card to go thru a turnstile and then pass the card back to the next family member.