I don't smoke, and most of my immediate family don't either, partly due to the example of my father-in-law who loved to smoke, and paid for it with his life. Combustion products (whether in smoke or in high-temperature cooking) are usually carcinogenic because of the formation of benzine (and similar aromatics) rings which interact with DNA and contribute to transcription errors. So anything that reduces the harm caused by smoke it generally a good thing. I know of some people who are vaping, and they seem to be quite happy with both the experience and the perceived relative safety. There is a move in a number of jurisdictions to regulate e-cigarettes and vaping products as tobacco products. Part of the concern is the interaction of the oils and glycols with the mucosa - the long term effects are not yet known. It may turn out to be a non-issue, or it may show effects that are currently not anticipated. I suspect that the relative safety will also depend on whatever additives may be added to the liquid, and keeping track of which one cause problems and which ones are safe will be quite difficult.
In some ways this is similar to the medical marijuana issue, where while the anecdotal evidence is pretty compelling, the science-based testing is lagging partly due to the lack of standardized delivery mechanisms, doses and purity. It is difficult to organize a truly double-blind testing regime unless one can get ingredients of known purity, potency and dosage (and placebo products that give the same experience without the active ingredient). And yet, that testing is what is needed to put THC in the same class as other regulated medicines.
@o hey tyler : Don't mean to hijack your thread, but discussions of the "cons" of vaping are not based on much evidence, so much better information is needed before we can say if vaping is truly an improvement over smoking or not. At least from my perspective, it's a better way than smoking, but then, there's also much that is currently not known.