Stevia is a naturally occurring herb (a friend once gave me a stevia plant; I chewed on a leaf and it was very sweet!). When the leaves are dried, the resulting powder is something like 240 times sweeter than sugar. Consequently, a little bit goes a long way.
Over the years I have experimented with using stevia in recipes. I have found that if I completely eliminate sugar from the recipe and replace it with stevia, the result is very bland.
However, I have found that stevia works synergistically with sugar to enhance its sweetness. So, if a recipe calls for, say, 1 1/2 cups of sugar, I reduce the sugar to 1/2 cup and add 1/4 tsp. of stevia. That seems to work pretty well.