Ringlights give very flat, 2D lighting for macro work in general. That is fine for medical macrophotography, but it doesn't look as good on 3 dimensional objects. Sometimes you want to light from the side, like in this shot of a Blackbelly Salamander -

For macro shots, I would much rather have a decent TTL flash that can be used off camera (either wireless or with a sync cord). Two flashes will give you even more flexibility.
I know that newer ring flashes give you the ability to turn off the tubes in certain parts of the ring (or only fire one of the 3-4 tubes), but I still think you would get very flat lighting.
To be absolutely honest, I would love to play with one to try and test it out, but I doubt I would use it for much herp photography.
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Chris Harrison