Just one word of advice, best to leave the box the phone came in in HK. Take out the phone, charger and manuals, etc and put in your bag. Last thing you want is getting taxed going back into China
Yes, definitely, as in HK, no tax!! In China, 17-22% tax, my friend owned an Apple franchise store in China, they got the stock from HK also.
the place is called sin tat plaza (先達廣場), phones are possibly marginally cheaper within hk as there is no sales tax, but in china it's at 5 %
plus in the mainland they usually strip off WLAN and 3G functions...