Update 2019: representative study of US youths found that using e-cigarettes as one’s first tobacco product was associated with more than 4 times the odds of ever cigarette use and nearly 3 times the odds of current cigarette use over 2 years of follow-up.
Dinakar & O'Connor, Review, The Health Effects of Electronic Cigarettes, NEJM 2016. 
Combes RD, Balls M 2015: On the Safety of E-cigarettes.
Excerpt from Fairchild et al.: The Renormalization of Smoking? E-Cigarettes and the Tobacco “Endgame”. NEJM 2013, Dec. 18, DOI: 10.1056/NEJMp1313940
...e-cigarettes are the subject of a public health dispute that has become more furious as their popularity has increased. Whereas some experts welcome the e-cigarette as a pathway to the reduction or cessation of tobacco use, opponents characterize it as a dangerous product that could undermine efforts to denormalize smoking. Already, Boston has applied workplace smoking bans to e-cigarettes. New York City and Los Angeles are poised to go a step further, prohibiting their use in public (including in parks and on beaches), though a similar proposal recently stalled in Chicago. This debate occurs as tobacco-control advocates have begun examining policy options for a tobacco “endgame” — the implementation of radical strategies for eliminating tobacco use globally.

Marketing campaigns for e-cigarettes threaten to reverse the successful, decades-long public health campaign to denormalize smoking. The chief advertising officer of one e-cigarette company has spoken explicitly about the “renormalization” of smoking in the form of “vaping” — the popular name for e-cigarette use. Even Big Tobacco dared not utter such words as the image of smoking was transformed over the decades. As information about the hazards of sidestream smoke was publicized in the 1980s and 1990s, the imperative to protect “innocent bystanders” moved to the center of tobacco-control efforts, and public smoking bans pushed smokers into the shadows. The once-widespread habit didn't simply become denormalized or marginalized; it became highly stigmatized. The pervasive became perverse.

E-cigarette advertisements, even as they denigrate traditional tobacco cigarettes, are challenging a barrier to television promotion erected more than 40 years ago. “Smelling like an ashtray is not the ideal aphrodisiac,” scolds talk-show host Jenny McCarthy, as she enjoys her Blu eCig. Actor Stephen Dorff, another Blu spokesperson and former smoker, similarly acknowledges that smoking is seen as dirty but adds, “I'm tired of feeling guilty every time I want to light up.” He implies that public health messages are paternalistic: “We're all adults here. It's time to take our freedom back. Come on guys, rise from the ashes.” On Super Bowl Sunday 2013, an NJOY e-cigarette ad seen by 10 million viewers declared, “Finally, smokers have a real alternative. Cigarettes, you've met your match.”

The tobacco-control community has responded to these messages with alarm. In 2009, the World Health Organization warned that e-cigarettes threatened bans on public smoking, which have been key to tobacco control. Similar concerns were raised by anti-tobacco activist Stanton Glantz and his colleagues: “Given the substantial research demonstrating the effect of viewing smoking in the movies on adolescent smoking initiation, the addictive nature of nicotine and the lack of regulatory assurance of their quality or safety, it is important to keep ENDS [electronic nicotine-delivery systems], and other similar products, from being sensationalized through the use of celebrity promotion or product placement in movies or other entertainment media.”1

These fears are compounded by data from the Centers for Disease Control and Prevention showing that twice as many young people experimented with e-cigarettes in 2012 as in 2011, although use of

tobacco cigarettes declined in the same period (see graph Use of Cigarettes and Electronic Cigarettes by U.S. Students in 2011 and 2012.). If e-cigarettes prove to be a “gateway” or “bridge” product, leading to an increase in underage smoking, that would represent a serious setback in the fight against tobacco-related illness. Invoking images of terrorism, two tobacco-control advocates claim that “Smoking bans and clean air advocacy are being hijacked.”2 Australian tobacco-control advocates Simon Chapman and Melanie Wakefield warn that something sinister is at work. The goal of e-cigarette makers is not cessation of tobacco use but “dual use”: e-cigarettes simply “capitalize on harm-reduction sentiment” to sustain what has become a private habit by reopening public spaces. They argue, “This could be a harm-increasing outcome when assessed against the status quo of ever-declining smoking prevalence.”3

In September 2013, 40 U.S. attorneys general called on the Food and Drug Administration (FDA) to act swiftly to regulate e-cigarettes as tobacco products. Dr. Howard Koh, assistant secretary for health, has urged leaders of U.S. schools of public health to join an effort to make U.S. colleges and universities smoke-free, which would include banning e-cigarettes....