As an ex smoker I occasionally turn to e-cigs in moments of weakness. I think they should be treated as a "giving up" aid and not a "permanent alternative" to smoking. I personally would never smoke an e-cig anywhere I wouldn't/shouldn't smoke a real cigarette. I totally support the airlines' view on this.
As for the legality, I imagine they will be legalized when they can be regulated. This will increase the price but it will also ensure that "vapers" are inhaling regulated liquid as opposed to cheap/nasty compounds manufactured by (some) cowboys. My concern about e-cigs in their current form is the content of the juice, not the nicotine content per say. Much of the juice is manufactured in China, a country where certain companies were willing to kill their own babies with cheap and nasty alternatives to milk products in order to make profit. I'm not having a deliberate go at China here but they have a long way to go before they catch up with quality regulations compared to Europe/America.