If the question is “how effective are electronic cigarettes as a cessation device?”, that's again a slightly complex answer, because there's a lot of anecdotal evidence. A lot of people will tell you that they have used electronic cigarettes to quit smoking. A lot of online surveys will also report that people are using electronic cigarettes to quit smoking, but if you look at trials that have compared electronic cigarettes to other evidence-based treatments to help people quit smoking, there's very little available.
Probably really only one randomized control trial of any rigour has been done, a New Zealand trial that suggested that electronic cigarettes were about as effective as a nicotine patch, but in that trial, both nicotine patches and electronic cigarettes were actually not very effective. But that's one trial and, for perspective, we can look at the thousands of trials that have shown that nicotine replacement therapies—patches, gum, and those kinds of treatments—are quite effective in doubling and maybe even almost trebling the likelihood of success.
I would suggest that there's an evidence gap even though there are some lower levels of evidence that electronic cigarettes can be helpful. The big thing that I think overlies all of this is that people like electronic cigarettes and people want to use electronic cigarettes. There is rapid growth in the use of these products, not in the way that nicotine replacement therapy is viewed, which is viewed with some lack of enthusiasm by people who are looking for an answer.
To answer your question, we don't really know how effective these devices are. That evidence is emerging.