I'm a little late to this conversation but thoroughly enjoyed reading the wide variety of responses. I was born in HK but went stateside early so don't speak Cantonese. But I do speak Mandarin. In the 40 plus years of traveling back/forth -- either for pleasure or business, I've seen a transformation in the local environment.

Back in the late 70's/early 80's when Taiwan opened up travel to China via HK, many of the retail areas [especially the jewelry shops!] learned quickly to speak mandarin. Behind the tourists back, they'd snicker but happily count the cash rolling in. Elsewhere you had to rely on Cantonese and even English was not as wide-spread as it is now.

Post turn-over, once travel from China opened up, this dramatically changed. Now Mandarin is a must secondary language - almost on equal status to English. Some 30-40 million tourists visit last year, and 80% from China....

As an example, I have a son who has lived in HK the past 7 yrs and recently just got his "citizenship/passport". He speaks English, Mandarin and shame on him - not a word of Cantonese, except how to understand mini-bus driver directions/instructions

If you're not Asian, any language skill is appreciated, whether in Cantonese or Mandarin. If you're Asian, they will always speak to you in Chinese. IMHO