Carl Root , Apr 17, 2003; 12:14 p.m.
I own it and would be happy to sell it to you. :-)
Seriously, although my old eyes have a hard time getting good focus, it allows you to double the size of the subject while maintaining the same distance to the film plane. Extension tubes get the same result by allowing a lens to get closer to the subject than its' minimum focus distance would normally allow. I use my Tamron 90mm macro lens most of the time unless getting very close to the subject is not possible.
The 2x teleconverter also serves as a conventional doubler as well, with the loss of light and sharpness that comes with all doublers.