In article <4a8m9h$lrl@panix2.panix.com>, paul@panix.com says...
> I recently bought one but can return it. A book I have by a prof. >Audubon photog. says he doesn't like them. A friend/pro photog says >they are garbage.(...)

As 2X converters designed for shorter lenses go, the macro Vivitar
is excellent (I have tried a few, and they are approximately equal
to the fine Nikkor TC201). As with all good short 2X converters,
they perform well on a few lenses, o.k. on many lenses, and poorly
on some lenses that they are physically designed to fit. I would
never use an aperture wider than f5.6 (effectively f11) with one,
and performance will generally improve with smaller apertures down
to about f11. The loss in lens speed is a serious disadvantage,
except in flash macro work. The speed loss plus the fact that these
are not designed to work with long lenses may be why the bird
photographer didn't like them. As to why your friend didn't......
I have found them wonderful for insect shooting at small apertures,
and for photographing setting suns and the moon, but otherwise they
may be of little use except to approximate a longer lens than what
you have. BTW, the short 2X's seem to be worst on the very lenses
that you would most like to use them with: longer lenses and zooms.
Used with understanding of their limitations, converters are well
worth owning. (I have five different converters, and use them often.)
Hope this helps.