I just got back and with so many places accepting Octopus it simply made sense. I carried some cash for the "high street" shopping where cash is king, but otherwise I was able to use the card pretty much everywhere. It sure beat carrying around coins and figuring out change.
Same experience for us in mid-January. I used cash a few times for shopping at street stalls (Apliu Street flea market and some tourist targeted stalls in Causeway Bay) and a CC for higher end stuff like a dinner buffet in a shopping centre. Thanks to this thread, my wife and I both used Octopus for Tourists on our iPhones and it worked flawlessly. Topping up via a US CC was also way better than the old way of going to a cash machine and then putting said cash onto a physical Octopus card via a machine at the MTR station.
This probably won't apply to most reading this thread but FWIW, the building that we stayed at was able to link my mobile Octopus card to my registration, so my mobile Octopus card also functioned as the "key" to get me into the building. I worried that this would only work with a permanent Octopus card (mobile or physical) but to my delight, it worked with my technically temporary mobile Octopus card.