Its quite common in cities with a big foreign presence or lots of foreign companies. There are some words that just don't translate well, and instead of spending the time to explain it, then just say it in English. It mostly happens between people who know each other well, and are confident the person they are talking to will know what they mean. For example, I have a lot of friends who do music, and a lot of the recording/electronic music software doesn't have a Chinese version, so whenever they are talking about that stuff they mix in a lot of English words, its just easier that way. Other times, people do it when its completely unnecessary just to show off and seem educated or international. In that context I think it just makes them sound stupid. In the majority of China, in day to day life, people just stick to Chinese. Somehow Hello, ok, and bye seem to have gotten mixed into their vocabulary but other than those I don't hear much English.

It's not really common, however; there are more and more Chinese want to learn how to speak English. Also, I think some people who go aboard or speak English during their work for a while may have this habit. They may not translate the words really quick. For my personal experience, sometimes I may say some English words to my parents unexpectedly (they don't know anything about English).

Not really, but in think if some people are good at English, u know

well,i usually say some english words to my schoolmates in my school life in shenzhen .but there are many "chinglish" words or sentences like 'mountain people mountain sea'（人山人海），yeah,wow ,ok,great and so on. --just for fun ^-^

no, it's not common