In summer 2005, I flew with Air China (Beijing-Chengdu) in their 737 Classics. Well, it may differ from 738 a bit, but it is gonna be similar.
The cabin was very narrow (3+3, 1 corridor) and the air in that particular flight was very STIFF. You may like to prepare iced water and wet towel for your comfort. It would be quite inconvenient for u to go to toilet (esp. if you sit in the window seats) since it's really narrow in Economy class. And mind you, not just Air China, but the 3 main mainland airlines have NO Personal TV sets in their Economy class. So it's quite dull, just watching the main screen (I don't remeber if they have that) and the in-flight magazine.
Cabin environment...Well, as 738s are not too old, it should not be too dirty. But the seats may be charged with electrons, i.e. you may feel like being shocked if you touch them sometimes. Food is not too bad, but don't expect much from them. As for toilets... they may be a bit smelly, but it's still ok.
Anyway, BON VOYAGE!
參考: Air China flight 1409 (PEK-CTU, 25/7) and China Southern flights, also my own experience
When you travel with any airlines from Mainland China, do not expect they would have PTV as none of them do for Economy Class...
As of the aircraft, it is a narrow body jet with a seating layout of 3+3... Personally I dont enjoy much flying with B737 because of the narrow cabin... Having said that, if they provide a good legroom then it could be rather comfortable...
Also, as majority of flights by Air China on flights to China will provide meals made in China on both direction (apart from early departure from Hong Kong)... So their size could be pretty small compare to most other airlines... Having said that, their taste are OK...