They might be fairly harmless, we don't know. The three biggest risks of smoking are cancer, emphysema, and heart disease. For a casual smoker, heart disease is by far the biggest risk. This is caused by the carbon monoxide, and eCigs have zero of this. There's also no reason to think that they cause emphysema. In terms of cancer, the majority of the carcinogenic effect of cigarrettes is thought to be caused by radiation rather than by the tobacco itself. So with eCigs you can easily wash off the radiation from the tobacco, or else just use synthetic nicotine or other drugs.
Since the industry is completely unregulated I'm sure these things are filled with all sorts of poison, but even still there are no currently known risks. And even if there are risks, which there likely are, it's not clear that they'll be any bigger than, say, drinking the occasional glass of wine. And it's also likely that once we figure out what these risks are, we'll be able to mitigate them.
I certainly wouldn't use these things today, but in twenty or thirty years they may well be a really good bet.