It's probably about as dangerous as living in any major city. Is breathing in anything other than air bad? Yeah, probably.
In comparison to smoking a bunch of plant matter, whether it be infused with toxins or not, can leave ash in the lungs which is generally bad; water vapour and proplyene glycol (which can mostly be metabolized by the body) is probably fine. Maybe.
Thing is they haven't been around that long either. Maybe Prop.Gly. to the lungs is cancer in a little tube.
Or maybe its completely harmless. We don't know for complete sure, and it doesnt help that regulation is a little bit whacko with this new product.
As a smoking alternative though, you really cant go wrong. I'd take a strong vape over a cigarrette any loving day.
I'd be a teensy bit wary personally, If I ever got into it, 'cause the nicotine might end up being the biggest problem with it. Just have to exercise control, which is probably the biggest downfall with any addictive substance.