For those interested here is a good review regarding Usnic acid
http://ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExSumPdf/UsnicAcid.pdfMore recent toxicological studies are listed here
http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=search&db=pubmed&term=tox%5Bsb%5D%20AND%20%28%7E125-46-2%5BRN%5D%20OR%20%22%7E%22%5BMH%5D%29As a point of clarification the anti oxidant systems that Flexington mentioned are all present in isolated cells. However some of them may be inactive due to depletion of co factors such as ascorbate and glutathione.
I am not sure if any study has shown a thermogenic effect at the levels given in most supplements. Increased dosage would presumably lead to mitochondrial uncoupling, and thermogenesis (with resultant fat loss), but that might also lead to toxicity.
If you eat a balanced diet and take your 1 or 2 a day vitamins anti oxidant supplementation does not have a major effect on your cells abilities to eliminate free radicals (superoxide dismutase, catalase) or to reduce oxidative damage (ascorbate, glutathione, etc).
So good luck to those supplementing with it. It may work, it may kill you, it may work and kill you. Most likely at the low doses recommended it will not do much of anything, but as we know a lot of bodybuilding is based on super pharmacological dosing, well outside the range of human research stuies - the label has never held many back.
My only suggestion is to be like the Beast, and research it yourself before deciding.
Best
Doug