Well, we could look at this a couple different ways. "Cool" as in "something that's considered socially acceptable and does not explicitly violate any rules," in which case, this'll need to be a case by case basis: per this Businessweek article probing New York chefs for their thoughts on in-house e-smokes, a few places (namely Momofuku) have already banned them outright.
Mostly, though, the chefs in question seemed to find e-smoking in restaurants inoffensive, if a little tacky ("They look like a prop I would’ve had in my raver days," Do or Dine's owner told the site). Which brings us to the second question here: even if it's allowed, is it "cool" in the sense that smoking an NJOY in between bites of ribs is not just a neutral act, but something that'll actively make people want to be exactly like you? Or sleep with you? Or at least, not shame you into leaving said restaurant and never coming back? Ehhh. Jury's sort of out.
I mean, it'll definitely get people to stare at you—if there's any one prevailing feeling people seem to have about e-cigarettes, it's that the glowing blue or red ends look weird and unnatural, especially in a setting like a restaurant where no one has been used to seeing any kind of smoking for the past ten years—but other than that, reactions are generally mixed (or indifferent). A lot of hardened smoke-smokers don't like it, and it does look sort of insane. Even when they first launched, I kind of assumed no one other than the most desperate-to-quit nicotine addicts would deign to be seen with them. But then, people like having something to do with their hands, they really like smoking, and they especially like smoking without feeling like they're doing anything that wrong (or having to go outside). They're everywhere, and at this point it's more notable to be at a party where there aren't a few people comparing notes on their respective e-cigarette brand and flavor choices than the other way around. It doesn't hurt that companies are giving these away for free everywhere you go, but you could say the same for Snus, and nobody's gotten onto that bandwagon. I don't know, I guess while we still live in a weird limbo where the city hasn't set down any official ordinances about them (a time that likely won't last too long), may as well do whatever you want with these. Every movement needs its pioneers, after all.
Follow Virginia K. Smith on Twitter @vksmith.