I have a Ray Flash ring flash that I got for macro work. Whenever I mention it to a photographer, they often say that they really like it for portrait work.
What are the advantages of using a ring flash for portrait photography?
The advantages when used as a main/only light include:
There are disadvantages too, unlike a softbox the light is unforgiving of blemishes etc.
The ringlight can also be used more subtly in a multi light setup to improve definition by throwing light into shadow areas.
Ring flash gives a very distinctive look. It virtually eliminates shadows on the face of the subject (ring flash shots are usually taken head-on) while leaving a soft, even shadow around the subject.