The Octopus card is a great service, especially for tourists. They have different branches throughout Hong Kong, but I used the one at the airport when I first arrived. The card is like a debit card, except that you don't need to sign for it and they don't ask for an ID. There's a $50HK deposit that you get back when you turn the card in. You preload the card and use the card as cash. You can use it for the metro, bus, ferries, stores, fast food places, almost anywhere. You put it up to the scanner and it deducts from the card. There's no signature or ID needed. Whatever you don't use, you can get back when you turn the card in.
The positive is that it's very convenient. I would just bring this card with me to the 7-11 by the hotel and just scan it. I didn't need to look for change for the bus or deal with the metro machines for transportation.
The negative is that it's treated like cash, so if you lose your card you won't get your money back. I put just enough so that it would last me a few days. When the amount got low, I would deposit money at 7-11 or at the metro stops. read more