It has been in use as a sweetener in Japan since 1972, with no problems. The argument against approving it for that use in the US is based on an animal study that fed HUGE quantities of refined stevia to, I think, rats or mice. The results of that study suggested possible carcinogenic or tumor causing effects. The argument they use to ignore the fat that Japan has had no problems whatsoever is to say that the Japanese do not eat as many sweets as Americans do, & it would be easier to consume a large qty in a typical American diet. Now we know that it's just BS, cause if they were so concerned Nutrasweet & Sacharin wouldn't still be on the market.
Anyway, some products you can buy in health food stores have started using it as a sweetener, I don't know if they have to call it a supplement on the label or what, & herbal teas sometimes include stevia leaf, I've seen it in Celestial Seasonings. When I use stevia, I use it in an unrefined form, or a partially refined form, like an extract (see my note above). There have been no studies, that I know of, linking the unrefined plant with any problems, plus it can be beneficial for the pancreas & digestive system, among other things, &, like most fully refined foods, you don't get those benefits from the pure extraction of the sweet-tasting components, stevioside & rebaudioside. Oh, I need to correct my earlier post about sweetness, the refined product can be that sweet, whole stevia is "only" around 30 times sweeter than sugar.
This website has info & recipes. There is a conversion chart, too, but it is specific to their products, don't know if it will help or not http://www.steviaplus.com/index.htm