I've written about that sweet flavor causing an insulin reaction. It's called 'cephalic response', where the brain has evolved to recognize actual sugars from foods but will react to anything sweet because it can't tell the difference. I've seen a little research but not specifically with stevia, so I don't know how true this is for that sweetener, or whether it's true for some people and not others.
As for agave, all this news about it is a little frightening, not because the news is so bad, but because it raises so many doubts and questions. I'm relatively certain that many of the agave products we educated consumers purchase are pure and are processed as indicated on the labels. But on the other hand, can we be certain? I suppose phone calls to the manufacturers can clear up a lot of the confusion. But this article also brings up the point that blue agave is in short supply, so environmental issues become part of the equation, too. So, do I want to contribute to an industry that's hell bent on eliminating this plant? But again, maybe there are responsible companies who are turning to farming agave and doing so sustainably.
For me, I don't eat a lot of sweetened foods and I've never gotten into the habit of using agave, so I'm just going to stick to my raw honey, a little dark maple syrup and soaked dried fruit for sweetening things. Those are my favorites. I also like molasses and purchased a bottle of organic molasses from a member of our co-op who'd bought a case. Turns out it's from Paraguay. Oops. I'll go through this quart (should take me about 10 years) and then try to find something more local.
In the meantime, where's that bottle of tequila? All this global food mess gets a girl thirsty.