From FontShop founder Erik Spiekermann
Avoid flush settings! Most applications create justified text by hideously stretching and squishing words and spaces. Note that it takes many hours of tedious work to typeset justified text that is truly well-proportioned and legible. For this reason, professionals prefer to use ragged-right composition, either with or without hyphenation, depending on how much line-length variation they wish to allow. This gives the text a more harmonious appearance and makes it easier to read, since all wordspaces have the same width.
If you are up for a history lesson on the origins and use of justified typesetting, ITC's U&lc tackles the subject quite well. And please people, there is no such thing at right- or left-justified type!