I am surprised that no one posted this yet.
Dependence levels in users of electronic cigarettes, nicotine gums ... - PubMed - NCBI
http://tobaccoanalysis.blogspot.com/
Some e-cigarette users were dependent on nicotine-containing e-cigarettes, but these products were less addictive than tobacco cigarettes. E-cigarettes may be as or less addictive than nicotine gums, which themselves are not very addictive.
Dependence levels in users of electronic cigarettes, nicotine gums ... - PubMed - NCBI
http://tobaccoanalysis.blogspot.com/