Despite having lived in China for over a year, I just figured this out today as I flew into Beijing Capital Airport for the n-th time. If you want to exchange money, do so AFTER going through customs. Outside of customs (as you turn right) there will be an Agricultural Bank and an ICBC both of which will give you decent rates and NO commission. Whereas the Travelex offices and others that are before customs (ie/ near baggage claim) will charge 40 RMB commission, though they will speak English at a much higher level... Not that it's really needed.

Hope this helps. Cheers