That depends on the kind of subject and its purpose. Ring light is flat and shadowless, so is is excellent for technical illustration if you are a botanist or etymologist, or something like that. It does not model a subject with shape well and tends to produce too many specular highlights in shiny subjects. But big ringlights have been used in fashion photograpy and editorial portraiture to some advantage.

George Lepp designed a two head macro bracket setup that would support a more creative ratio lighting that allows one light to be greater than the other in output favoring better modeling of shape and more aesthetic macro photography, particularly of subjects like bird and flowers.

Auto flash exposure control, TTL, can be a bit tricky with ring-flash in part because the light is so flat and low contrast and agin because shiny subject's specular highlights can result in underexposure. Usually a ring-flash made by the camera manufacturer performs better with TTL exposure control.

Not sure about David's post. With the Sigma and Canon ring flashes, each are really two flashes; you can dial down one or both, or turn one off, if you like. You can have various ratio's and if you do get one, I recommend your camera brand or Sigma's, if you are looking at cheaper alternatives, save your money and get a decent ring light.