On my last trip, I spent quite a while wandering around shopping with friends, and checking out the exchange rates. They were all over the place. Where the going exchange rate was 7.7 or so per US dollar, I saw rates as low as 6.45 and as high as 7.4 or so in street booths.
The best exchange I got was in Wan Chai, right here:
http://maps.google.ca/maps?q=7+Gloucester+Road,+Wan+Chai,&oe=utf-8&client=firefox-a&ie=UTF8&hl=en&hq=&hnear=7+Gloucester+Rd,+Hong+Kong&ll=22.276612,114.173277&spn=0.001996,0.003962&t=k&z=19
It's close to the Wan Chai subway, in the low rise building between the subway and the playing fields. The rates there during the summer were signficiantly better than anywhere else (including the airport).
If you can get close to actual exchange rate before leaving on a trip, do it. If you have an ATM card, you should get decent exchange rates on that. Just remember that you may have to change your pin code to a 4 digit pin as some asian systems do not accept longer pin numbers (or don't properly transmit them for approval). You will typically lose about 3% or so this way, sometimes more if the ATM fees are high.