I'm visiting Hong Kong, and I have AMEX. Is it well-covered or should I prepare Visa/MasterCard?
I would suggest you to prepare Visa / MasterCard as well.
From my general perception, AMEX is accepted in bigger merchants only.
Visa / MasterCard is well accepted around the city, including almost all restaurants & shops in malls.
Also, it is also good to know that Octopus card is also well accepted for smaller amount transactions, including transportation, convenient stores, supermarkets, etc.
Finally, some smaller merchants accept cash only.