To Neha's point about people moving here to try to get a piece of China, while that may be true now, it isn't going to be that way for long. What is happening already is that the focus is slowly shifting from Hong Kong north to Shanghai. Why use HK as a gateway to China when firms can actually now be located in China? Very large US law firms, banks, etc are not opening offices in Hong Kong but they are going straight to Shanghai and Beijing. If we stayed in HK, we'd have to relocate to Shanghai in a few years for this very reason.
Also to whoever was saying regardless of pollution, expats will come for lower taxes, etc....it obviously depends on individual circumstances and our overly complicated tax system, but in general US citizens pay virtually the same amount of tax as they would in the states because we are taxed on our worldwide income. And to be honest, I've heard from friends here recently that some US firms aren't offering the same kind of lucrative expat packages. They are still good, but nothing like they used to be.
Slightly off the pollution topic, but even more incentive to clean up the air I think.
Enjoy this lovely day. I can see blue sky!