Read a very interesting article in The Age today:
Stevia | Is Stevia Better Than Sugar?
It appears the final product is 'bulked up" with other carbs that are calorific.
That's if you buy the stevia blends ... the granulated stuff.
eg. Natvia is stevia bulked up and blended with erythriol, which is a sugar polyol of the same family as sorbitol, mannitol etc.
Erythriol is the least evil of the sugar polyols in the sense that is is fairly benign and does not cause GI issues (passes right through you unlike the others).
it is also not particularly calorific.
Some stevia blends are bulked up with other stuff although I've not seen them on aussie shelves (but I know Equal have brought out a Stevia blend so maybe that's one).
If you buy pure stevia extract, it's just that, pure stevia...alternatively, stevia in the raw or powdered stevia leaf. you can get these at health food stores and Professional Whey also sell it (a particularly good one with no bitterness).
Ultimately, it's not sugar vs stevia. Sugar in true moderation is not harmful. It's just that most people have no real idea of how much sugar they consume on a daily basis.
Stevia is not a wonder product but it has been used for centuries in south america, it's apparently pretty safe given that, and I use it sometimes when i want to keep the carb count down on recipes.
You are better off using real sugar, maple syrup, stevia, coconut sugar etc than using most "sugar free" processed products that contain sugar polyols, sucralose, aspartame etc.
My gastro guy says he keeps telling patients this (it's the real food vs test tube food debate) but is disappointed most people just don't get moderation.
I think these articles are a bit silly at times .. with the whole A vs B ...