So, here I am, reading this article on the FDA proposing rules on “e-cigs”, and come across this:

Some smokers use e-cigarettes as a way to quit smoking tobacco, or to cut down. However, there’s not much scientific evidence showing e-cigarettes help smokers quit or smoke less, and it’s unclear how safe they are.

… Tobacco company executives have noted that they are eating into traditional cigarette sales, and their companies have jumped into the business.

That sure sounds like economic evidence that they help people to smoke tobacco less.