Impolite. Actually it is an out-of-date saying and you only see in costume dramas. You can use it to your friends playfully instead of "Chao!," but you never can say before an older person nor someone in business.

さらば or さらばだ was used between Samurai(feudal worrier) as a proper greeting of farewell. But after that, this word remains as a written language. We still use it for a title of a novel or a movie. "A Farewell to Arms" is translated as 武器よ　さらば. Comparing to the ordinal さようなら, さらば has flavor of graveness,solemnity. You can not use this word in your verbal communication.