Absolutely not. Forget the fees -- the real money in foreign exchange is in the buy / ask spread, and travelex is one of the worst. Your "free miles" would literally cost you hundreds of dollars.

Use ATMs, or bring US currency and exchange in Hong Kong. You should be able to get exchange rates that are fairly close to the market rate from a number of the independent exchange places -- there are plenty either in Kowloon or on Hong Kong Island itself.

If you end up with leftover HK currency, plan to exchange it back to US dollars before you leave, use it to pay your hotel bill, or save it for your next trip. The rates you'll get in the US will be atrocious.