The 'Strobist' site has a wealth of information on ring-lights, ring-flashes, and ring-reflectors.
What to look for, what to avoid, expectations as to performance, etc...

Below are some sample posts...there are many more at the Strobist site.

http://strobist.blogspot.com/2007/11...resources.html

http://strobist.blogspot.com/2009/10...es-abr800.html