I'm an ethnobotanist, so this posting made me prick up my ears. Claims for plant-based medicines can be dicey sometimes, particularly with commercial spices which can be derived from a number of often-unrelated species. This, in turn, has an impact on the chemistry and pharmacological activity of such preparations, which can vary widely and sometimes can even be dangerous.
As far as cinnamon goes, commercial cinnamon ranges from the high-grade, expensive cinnamon derived from
Cinnamomum zeylanicum to the lower-quality, cheaper Chinese cassia, derived from
Cinnamomum aromaticum. We're in luck here, because the cinnamon that appears to have been tested clinically is the less expensive Chinese cassia. Nevertheless, when it comes to spices, you get what you pay for and the cheaper cinnamon is often adulterated with fillers like walnut hulls, etc. This may account for why some people report an insulin-lowering effect and others do not -- the quality of the cinnamon varies.
More is not better here, either. Above 1/2 tsp a day, some sensitive individuals have reported side effects, most often caused by irritation of the mouth and stomach. (Check with the HerbalGram website for good info you can trust; other sites can be sponsored by manufacturers).
Finally (if you haven't gone to sleep and are still reading my pedantic post), similar insulin-lowering benefits have also been found from the aromatic culinary herbs and spices bay leaves, cloves and turmeric (often found in curries). OK! Lecture is over. There'll be a quiz Friday.