I would advise, if you're living and working in Hong Kong, to learn Cantonese with all deliberate speed. You'll find lots of doors will open up to you when you can speak the local language, and, I can attest from personal experience, that most local Hong Kongers can't speak very good Mandarin Chinese.
I went down there on holiday and began every conversation with Mandarin and got back mainly English replies. I went into a shop and began speaking Mandarin, the shopowner was doing his best to try and speak with me in broken Mandarin, after a minute or two he asked which country I was from, I replied the US. He then blurted out in English, "I thought you were from a non-English speaking country! Why don't you just speak English? My Enlish is SO much better than my Mandarin."
Now, whenever I travel to HK, I don't even bother with Mandarin.
Besides, learning things that are related to your needs is one of the best ways to stay motivated.