There are two factors you'll need to consider:
Will you have network coverage (in order for the app to download data)?
Will the program have detailed maps of the areas of interest?

From what I can tell, the major mapping providers TeleNav and NAVTEQ do not have detailed data ready for China. In 2004, NAVTEQ and NavInfo partnered in a joint venture called NAV2 to begin providing such data.

As for Hong Kong, take a look at Nav4All, which has good support for that region. Nav4All also noted support for China "soon", so maybe they will get access to the NAV2 data.