E-cigarettes definitely need more study, but it will take years before we really know how dangerous they are. Compounding the difficulty is that e-cigarette users tend to smoke differently than cigarette smokers. A cigarette smoker smokes one cigarette at a time, but with an e-cigarette, there's no beginning or end. It's likely that e-cigarettes are safer per puff, but are puffed more often.
Also, right now, the people most interested in e-cigarettes are smokers already, but going forward, will e-cigarettes provide a point of entry for people who otherwise wouldn't smoke?
It's a confusing debate, but one worth having.