There are certainly a lot of misconceptions about Stevia. I had never even heard of the herb until a few years ago. Now that Stevia is becoming more mainstream, there are a lot of different varieties available. Most have fillers in them to make it measure for measure like sugar. The fact is, though, that Stevia is 250 to 300 times sweeter than sugar, so it only takes a small amount of pure Stevia to equal the sweetness of sugar.
My favorite brand of Stevia is SweetLeaf. It is an all natural Stevia, with no fillers. There is no bitter aftertaste. You may need to modify your Stevia measurements if you are using another brand of Stevia.
Here is a great resource for any questions/concerns you may have about Stevia.