Hi Rachael,
I just returned from the July 26th China tour. Air China were really good, cabin crew are friendly, refreshments are good with the added bonus of being able to get up any time and go and help yourself to snacks and drinks (including alcohol) from the galley at the back. The plane was not too big but you get 33" seat pitch as standard in Economy which is more than what most other airlines offer making it spacious and seat-back entertainment with at least 25 films to choose from as well as music, TV shows and documentaries.
Sneaky tip though log on to the air china website and sign up for their free air miles card as you will earn an awful lot on the china trip - just make sure you present it at check-in every time you go through including for Xian, even if you don't think you'll fly with them again the points are still valid for other airlines in their Alliance.
The China trip is fantastic don't let other reviews put you off for every one person who complains there is another who praises :)
Helen