Amongst native speakers of English, "sayonara" is one of the best known Japanese words, at least for words that don't fill a semantic gap ("sushi" is derived from a Japanese word but fills a semantic gap). It's sometimes used in English when saying goodbye, usually in a casual context.
And yet, I haven't really noticed the Japanese word さようなら being used in real life, with a couple of exceptions.
One is when Japanese people are talking with English-speakers. One case I can recall is a Japanese teacher using it at the end of most of her lessons.
The other instance seems to be when Japanese has re-imported the word from English, and written it in katakana. For example サヨナラゲーム, or the English "Sayonara sucker" in "Wreck-it Ralph" being subtitled in Japanese with "サヨナラ[something]" (if I recall correctly).
One possibility is that in Japanese, the word さようなら is only used in formal contexts, which I haven't encountered.
Is さようなら still used in real life? If so, when is it used?