Should you get an Octopus transportation card?

The Octopus card is a prepaid card used mainly for public transportation (MTR, buses, ferries, minibuses), but you can also use it to pay at convenience stores, supermarkets, restaurants, and more. It's more widely accepted than credit cards and can be very convenient. 

You can choose either a physical card or a digital version on your smartphone. (Note: Some visitors have had difficulty setting up the digital version, so the physical card is usually easier.) You can “top up” (add cash) to the physical card at any convenience store—but with cash only, not by credit card. For iPhone users, here is info about the digital Octopus.

Even if you prefer using your credit card for transportation, the Octopus card is helpful as a backup, especially at small local shops, restaurants, or on minibuses, which sometimes accept only cash or Octopus. (Adam sometimes has extra Octopus cards to lend to visitors—just be sure to add money back onto the card afterwards.)

When leaving Hong Kong, you can return your physical Octopus card at the MTR station to get your deposit and any remaining balance back.

Should you get a "Tourist Day Pass" for transport?

While the all-inclusive day pass might sound convenient, it's usually not worth the money. Most visitors don't take enough trips in a single day to justify the cost. It's easier (and cheaper) to just pay as you go with an Octopus card or credit card.