The supposed advantage of a ring light is its shadow-less nature and
there are some pictures for which this may be desirable.  My preferences are
for more naturally looking illumination.

As far as equipment is concerned, the ring light I have is an ancient
(Soligor IIRC) not-completely-automatic unit that had to be used manually
with several test exposures before I could get decent lighting using the
D70.  I never did try it with either the 35mm film cameras or with the
2-1/4" square because of the uncertainly in the exposure.  I suppose a flash
meter would make all of that moot.

AFAIK There's only one ring light which works fully automatically with my
Nikon D70 but several units (including a Nikon) that work with my film
bodies (N90s & F100).  Since the Sigma ring light is ~$350 and the
off-camera cord and Sto-Fen diffuser totaled about $50, I didn't see much
reason to purchase the new ring light since I could get almost the same
effect by holding the flash right adjacent to the lens barrel.

   Norm