Mine has worked for years. Just keep it. That's what I do, I keep it with my passport and the odd currency I accumulate on my trips that I don't bother to exchange back on the way out.

When you exit customs at the airport, there's a desk right at the exit (before you clear that area where all the people are) on the right that sells the octopus cards. I just put 200 - 300 on it, which covers my Airport Express round trip and usually whatever extra trips I do on bus, train, ferry. You can check your balance/reload anywhere/anytime. In that same area, there's some ATM machines also, so first you will pass that desk, go to the ATM machine, get 500 - 1000 or whatever and go back to the MTR desk and buy/reload your Octopus card.

There's a bunch of places where you can pay for stuff with your Octopus card now. There's even a web site for it. (Quokka posted a link to it in post#2). It's a separate third party payment system.

It's a great system. (both Octopus and the HKG transit system). Enjoy.

-David