Air China isn't bad, we did the Beijing stopover (well, the first part of the trip was a 7 day tour in Beijing and then to HK) and to be honest you don't expect much from it. They are slightly lenient on the weight of your check-in baggages (moreso than Air Canada - but YMMV). The food is decent. Entertainment choices are pretty decent as well.
The only problem was when we were flying from HK back to Beijing for the stopover, they used an aircraft that was not fitted with the in-seat entertainment system. The whole flight they showed The Social Network
Other than that, the air hostesses were pretty cute, albeit their english is very limited, but that is expected.