I'm loathe to carry foreign currency and attempting the process of obtaining it. As a result, I decided to use the Octopus for Tourists app - yes, you can't charge in HKD, but the DCC seems relatively whatever if you think about the exchange rate fees for ATMs and/or cash changing. 150 HKD came out to only about 1 USD higher than it should have been (20 vs. 19). The readers at North Point and Admiralty had no issue with Express Transit on my iPhone.