I'm certainly no HK expert but I just returned from my fourth trip.
If you do pick up a local SIM, Google Maps makes it really easy to use the buses, which I often found to be faster than the MTR once you factor in walking to/from the station on either end.
I've had trouble with my Schwab ATM card in one or two places in HK, notably the ATM just before the Airport Express platform at HKG to the left of the ticket machine. IIRC that one doesn't actually take Visa cards.
I also found OpenRice and Foursquare great for finding good places to eat/drink when walking around. The former for local food, latter for Western. Foursquare led me to a great hidden bar in Wanchai that I've returned to on every HK trip since. The bartender recognizes me on sight at this point when I return every few months.
