I have taken the Air China LAX-PEK route quite often, as this is the best option for me to get back to China since I live close to Beijing, and Air China is the only carrier flying this route (thankfully American will start flying this route as well). I always fly business class on this route because there are too many crying babies on this flight and I want to have a proper sleep on the flight.
Air China uses 787-9 and 777-300ER on this route, with 2 787 and 1 777 daily. Before Air China received its 787, it is 3 777 daily. I have taken both planes.
Air China has identical 2-2-2 business class on 77W and 789, but the 789 is slightly narrower, making it slightly uncomfortable compared to 77W. I don't know why Air China and some other carriers are still embracing the 2-2-2 business class in 2017 (cough Lufthansa, even air berlin is better, offering 1-2-1 business class). Ok back to the seats. The seats themselves are decent, are flat bed on both planes, and the table is a decent size. Width is standard business class. Good legroom. Not the best business class, but definitely not bad. The IFE is the same on their 777, 747-8, and 787, and they offer a good selection of Chinese and Western movies.
Now let's talk service.
Air China uses the Star Alliance lounge in LAX, which is a very nice lounge. Food is great, everything is great. However, their lounge is Beijing T3 is not very good. The airport get pretty hot in summer, because PEK does not turn on AC for some reason. Food is mostly Chinese food, and some of them are quite nice. Not a lot of western choice thou. The foot Air China has good service if you understand Chinese, but most FA know little to no English. Sometimes they can be quite rude, and sometimes they are quite nice, depending on what crew you get. There are two meals, but the food is subpar compared to other business class food. There are Chinese and Western choices. But the problem is delay. Chinese carriers have a lot of delays, and this route is no exception.
Overall, Air China LAX-PEK is a decent product. American is opening the same route thou in a few months, so I would like to see what would happen in the future.