Most retailers in Hong Kong do not accept RMB.
The 7-11 stores* have an enlightend policy where they will accept RMB at HKD face value and give you any change in HKD. As HKD are worth less than RMB they make a profit of the exchange rate on the transaction.
The current exchange rate is 1 RMB = 1.25114 HKD (April 2013), so the 7-11 stores are making an additional 25% profit for their kindness.
Some other sellers MAY accept them but my recollection is that most will not.
Trying to use HKD in China is about impossible.
Note that in most of Asia NO commission or flat fee is payable. The rate offered applies $ for $ or whatever. In eg Australia they charge an iniquitous up front fee and changing enough to get eg $20 for a cofee may give you only $A5 or less. Utter thievery.
While, as uncovery says, there are a very large number of money changers in HK, I have found that banks and specialist big-business money changers offer poor rates and that the best rates of all are offered by a small number of small businesses if you know where to look.
I stay in Mong Kok when I am in HK (noisy, busy, cheap accommodation, more "authentic" than many areas, fun) and have found a money changer who invariably offers superb rates - about 0.5% off the actual exchange rate either way. Stunning. Location below. As I wander the city I note the various rate offered and in quite a few visits no other money changer has bested these ones. Taking subway to Yau Ma Tei (quick cheap ride from city centre) and exit at A1 and walk West along Pitt Street and you may be at their door - or, see below.
Rate assessment: You can instantly determine how good a rate a money changer is offering by asking their cash buy and sell rates between two currencies (or buy rate each way or sell rate each way, which gives the same result.)
The mean "markup is ~= ($buy-$sell) / ($buy + $sell)
Close enough you can use buy-sell difference / $buy/2
( = same as buy-sell difference / (2 x $buy))
eg if some one is trading HKD-RMB, if they offer
1.2 HKD per RMB if buying RMB (or selling HKD) and
1.3 HKD/RMB if selling RMB (or buying HKD)
Then markup = (1.3-1.2) / (1.3+1.2) = 0.1/2.5 = 4%
The simpler method gives (1.3-1.2)/1.2 / 2 = 0.1/2.4 ~= 4% close enough.
Purists will argue that the true rate is square_root($buy x $sell) but, again, the result is not much different.
For any given currency pair just look at the buy-sell difference. The smaller the better.
ForEx who seem to have booths in all airports and elsewhere offer consistently very terrible rates in my experience.
Western Union are not usually very marvellous.
Hong Kong's cheapest rates:
Please excuse degree of uncertainty. I will have photos but probably not findable in your time frame.
Location is "probably" where two red lines are. I start from Mong Kok proper near subway just above top of picture. Walk down Nathan road and after a few blocks drill in sideways.
You can spot them by the queue !!! :-).
Two cashiers AFAIR opening onto street. Two small fast moving queues of happy customers changing all sorts. About 2 or 3 doors along further off Nathan Road on the same side is a Western Union office with 3 cashiers and NO customers !!!!!! :-).
Fast, polite, professional / business like. No Chinese language needed.
"7-11" stores are unknown in my country (NZ) but are much the same as 7-11 stores in the US - corner stores selling food, groceries, drinks etc. ).