From a health standpoint, Chinese fast food is some of the best in the world. It makes minimal use of meats, with vegetables making up most of its bulk. It's a good idea, however to avoid the sweet-and-sour sauces in favor of soy-based varieties.
No one is likely to confuse fast food with haute cuisine, but your fast food experience will be much better if you treat the service personnel as you would the personnel at any restaurant. Be polite, accept that the servers are humans and occasionally make mistakes, and order quickly.