I bought a macbook in HKG and it was slightly cheaper than in the US. You save on the sales tax. One thing I've found is to make sure you ask for a discount. From experience, I've gotten 3 to 5 percent off or no discount, but bundled with software or mouse.

Mobile phones on the other hand, are definitely not cheaper than in US, as all phones in HKG are unlocked. You do get to trade your old phone for a new one though. Also, the selection is much better than what's offered in US. I had a friend who came to the States on holiday made a comment on how old the phones are, with the exception of the iPhone. Even that he said, there were better phones out there than the iPhone for the price.