The Octopus card is a must have for travelers and locals alike. It saved us time as we didn't need to calculate the changes. Just make sure you don't lose it.
We swiped it in the subway, at the Star Ferry, at the Peak tram ticket counter, at 7-eleven to buy water, and even at McDonald's! The standard Octopus card for adult costs HKD$150, with a refundable deposit of HKD$50, less an administrative fee of HKD$7 if you return it within 3 months after purchase. We did not choose the Octopus for tourist because it wasn't worth it in our case.
The HKD$50 refundable deposit covers for possible negative value. For example, I once forgot to reload my Octopus card, and upon exiting, my swiped card showed a negative balance. If I remember, I didn't get out of the station as I had to reload my card. The MRT counter gave me an additional card to exit, as he forgot to top up the negative value that was taken, otherwise I would have double paid.
The minimum reload amount is HKD$100.