Why I dislike ring flashes
One of the most frequently asked questions in this website is why I don't endorse the use of ring flashes. I guess people keep asking because these are supposed to be a macro standard equipment and they see nothing here about them. Well, I do not use ring flashes because I don't like their output. They are designed to produce a flat, shadowless lightning which is very convenient for coins, stamps and the kind, not in my opinion for animals. In nature there's a strong light up there, thanks to the old Sun, and there are shadows indeed.
So I always try to have two light sources in my pictures, a strong one from above (mimicking the Sun) plus a weaker one somewhere laterally to fill the shadows and make the textures pop up. In macro I only use cloudy-day even lightning when shooting insects within wider landscapes; and then, ring flashes are useless because they are not powerful enough to reach the background. Actually, that's the reason for these flashes to produce so often dark backgrounds.
Courtesy Nikon Inc. Additionally, as they are placed so closely to the optical axis, ringflashes tend to produce nasty circular reflections on shiny subjects such as some beetles, or water drops. Worst, modern ringlights are not circular anymore, but rather two side-by-side bulbs. If you are lucky, some brands give you 4 tubes by adding two more sources below and above the lens; but with Nikon's SB-21/29 series you have only 2 small lateral bulbs.
Such a lightning is not uniform and shadowless anymore, but there's a tendency to produce symmetrical double shadows, which are the ugliest thing you could put in an image. See at the right an example of this phenomenon, which is dealt with in the brackets section too.
Thus, instead of using a ring flash, I feel much better with 2 separate flashes mounted on a good bracket, that will allow this kind of lighting (should I want it) plus many additional options, at a much lower price - and enjoying more available power.