Would like to know about Octopus Card in Hong Kong.

We are family of four planning for HK trip by the end of November. I read in the some of the reviews that taking octopus cards are very useful.

I would like to know if these cards are useful in travelling to Victoria peak, Lantau island, Macau, Disneyland in MTR??
And from where can I get these cards from ? And for how much HKD should I buy these octopus cards for 4 days and 5 nights?