I've never changed money in TST. It's more a matter of bad reputation than anything else.

You can head up to Mongkok or over to Causeway Bay or just about any major shopping area: there are plenty of small exchange places around. Most cater to Mainland visitors and post RMB/HKD rates in the window, but they also do USD/HKD. Rates are almost always posted on the back wall, so it's not hard to eyeball two or three shops and get a feel for the right rate. As I said, there are a small number of shops known for rip-off rates -- usually the spiffiest-looking ones -- so check the posted rates very carefully.

Banks will charge fees and are not worth the hassle.