I've flown JFK to PEK (and back) ~10 times. My last flights were by far my best international flight experience as Air China recently (last couple years) upgraded this route to 777-300ER planes. The biggest benefit of this is the provided power plugs between seats.

If you're more interested in who you'll be sharing the cabin with, expect to be surrounded by Chinese. Easily 8/10 of these flights have included infants screaming for hours (thankfully this is drowned out by airplane noise but I'm sure would suck if seated nearby). Expect "mainlander" habits/manners.

Overall though, as you can see by my exclusivity with this airline, I definitely recommend them. Their pricing is great, the food is mostly edible, the attendants are capable, it's the shortest flight around (for JFK to PEK), the planes are brand new, and power plugs! Just bring a laptop/tablet with tons of movies/tv to watch