There is no real evidence about lots of things when it comes to e cigarettes because so little research has been done, but time will tell the story. After all, for 50 years the medical and scientific communities instisted that smoking tobacco was not addictive and was even good for you.... and Big Tobacco is still resisting the fact that cigarette smoking causes lung cancer.
That said, certainly the vapor from e-cigs seems less harmful than cigarette smoke both to smokers and innocent bystanders, but the fact is that the vapor contains chemicals... mainly, propylene glycol. Propylene glycal is used as a solvent and emulsifier in pharmaceuticals and in such diverse products as airport de-icers, massage oil, hand sanitizers and anti-freeze. Its effect in large amounts on human beings over time is not known at the moment, but like I said-- time will probably tell.