Long term safety is unknown at present. Use of the e-cigarette does not involve burning of tobacco and inhalation of tobacco smoke as occurs in cigarette smoking; therefore the use of e-cigarettes is expected to have a lower risk of disease and death than tobacco smoking. Introducing appropriate regulations will minimize any potential risks from e-cigarette use.

E-cigarettes have the potential to reduce the enormous burden of disease and death caused by tobacco smoking if most smokers switch to e-cigarettes and public health concerns are properly addressed.

These concerns include that flavours might be attractive to children and promote use among young non-smokers, that labelling does not reflect content, that they are being marketed inappropriately, and that they may weaken tobacco control efforts as they could be used in places where tobacco smoking is prohibited, or could promote use among smokers instead of quitting and, ultimately renormalize tobacco use in countries with advanced and successful tobacco control efforts.

More research is needed to examine these concerns.

Update: IARC endorses the World Health Organisation (WHO) statement based on the current scientific evidence: 

WHO: E-cigarettes: how risky are they? 

The link to the WHO statement posted on 29/01/2020 by the IARC provides recent information related to this Question and Answer of the European Code against Cancer posted on 14/10/2014. The European Code against Cancer Working Group on Tobacco developed this Question and Answer but had no involvement in the development of the WHO statement.