A worldwide survey of more than 19,000 e-cigarette users, performed by our group, found that 80% of participants quit smoking, while cigarette consumption went down from 20 to 7 cigarettes per day (median values) in those who continued to smoke (unpublished data).
http://www.bmj.com/content/347/bmj.f5780/rr/664916
http://www.bmj.com/content/347/bmj.f5780/rr/664916