I believe it is safe.
It's been used by the native population of Paraguay (I believe) for centuries, and in Japan for decades, and no ill effects have been noted.
I think that, like with soy, studies showing problems with it are based on the experimenters using isolates or extracting elements from the Stevia, and whole stevia is metabolised in a different way.
I certainly think it's had a better record of safe use than Splenda or all the artificial sweeteners.