I posted this elsewhere, but it is my feelings on the subject:
"Before I came to HK everyone I spoke with told me to learn Mandarin. When I got here people told me to learn Mandarin.
Now after taking many Mandarin courses and self study I find it almost impossible to progress much further. I hear Canto everyday all the time yet I am learning Mandarin? I find it frustrating not being able to speak w/ coworkers and friends when they launch into Cantonese. So I am now turning my full attention to Cantonese.
If you are going to be here a long time and don't have mainland China ambitions then I say learn Cantonese. It will help immensely here."
It is in this
thread that you may want to take a look at.