First of all your octopus card would be invalid for use in Macau.

But as for getting around HK, yes its a good value as you don't have to undergo the headache of carrying heavy coins and the amount required for most public transport is slightly less than if you paid cash. For example if you would travel the entire "Island Line" from Sheung Wan to Chai Wan, it would cost you $7.5 with cash for a single ticket journey, but if you have an Octopus, you would only be deducted $6.8 from your card balance. The Octopus card also makes for a good tourist souvenier.

To choose your card type as well as pricing:
http://www.octopuscards.com/consumer...bal/en/map.jsp