Hi -
I grew up in a holistic/organic household so I guess I knew about things like Stevia long before it was heard of in the mainstream. A few things I can tell you that I've picked up over the years (but haven't researched): it is made from the leaves of a plant, so it is indeed an herbal concoction, powdered or put into liquid suspension. The plant is common to South America and has been used as a sweetener in the Andes for thousands of years, apparently without any known side effects. The only known use for it, medicinally, is that the Paraguayan Indian men chew the leaves for birth control. I dont' know if it has proven effective or not, for that!
Stevia is HIGHLY concentrated - a dusting is all you need for a cup of tea or coffee. But it has been shown to be safe for diabetics and for others watching their glycemic indexes - because it is not really sugar, it is just an herb that is sweet in taste. The other thing to be cautious of is what I learned about carrying it around in small vials in my purse - it looks a LOT like coke
And who wants to be held up for hours while they test it to find out its not, eh?
Because of its powdered concentration, a small bit will last you for ages - and it won't work correctly in baking. Thinking about the chemistry that takes place in baking - that makes sense, since its really not chemically composed like sugar. If you just need something to dust in there for sweetener - then it works great. The bad aftertaste really only comes from over-using it - too much per recipe.
Hope this helps.