If you google stevia safety, you'll find many more studies that show it is safe than the few that cast doubt on its possible harmful effects. One study is not proof.
Also, you have to make sure the research has been accepted into a peer-reviewed publication. That ensures that they followed correct experimental procedures. If they don't the results can mean anything or absolutely nothing, depending on how it was done.
Here's one that cites some claims for the sweetener's safety:healing.about.com/cs/uc_directory/a/uc_stevia_jones.htm
This one states that it has been used for years by several groups of people with no known harmful effects.
If you like sweetener, I would certainly use it over sugar, especially to excess. We know that being overweight is definitely harmful to our health.