The Canon MR14EX is a good portable ring flash, as Jonathan says it's not very good for groups though I have used itwith a 28mm lens for three at a time.
The best thing about it is you can carry it around all the time and with a bit of care get almost studio quality light anywhere, I use mine when I'm not getting any time with the artist.
Here's a quick one with the Mr14EX:
( Eos 1V - 100 Macro on Kodak 400CN )
Red eye is a problem with the little rings, but isn't to hard to touch out .
The big ring flashes cost a lot more then the little Canon, but have much more power and will cover much larger areas, the Pro Photo has a limit in the number of frames you can do per minute without blowing the tube and is fragile, they need to be packed and used with care.
I use a Pro Photo ring flash when I have more setup time or am working in a small space, they're very good for bringing detail out in the clothing, and the shadows around the subject stand them out of backgrounds.
Here's a shot done in a small room on a small background:
( Eos 1DSII 50Macro )
I think ProPhoto have just put out an updated model that has no limit on flashes per minute and also a modelling light..