Ive been doing this for a while and while it does work very nicely there are some drawbacks (at least for us Canon users).
You cant control the aperture. So the DOF is VERY narrow. But sometimes this makes for wonderful images.
You need lots of light. Without a strong source of light it is very hard to find the subject and focus on it clearly.
All that said, I am glad I done it. I haven't scraped up the funds yet to buy that 100mm Macro so until then I'll just keep going with this.
Here's a couple images I came up with:
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Canon EOS 7D
EF 70-200
L f2.8 IS USM, EF 17-40
L f/4 USM
100mm f/2.8 Macro Lens, 50mm f1.4, SLR Zoom Gorilla Pod, Cactus V4's, Speedlites 580EXII & 430EXII, Manfrotto 190XPROB w/496RC2 Ball Head
My flickr