I use the term "electronic cigarette" under only one circumstance: the person asking me has a lit cigarette (or I know that they are a smoker)
Otherwise, it's "Personal Nicotine Inhaler". Yeah, I know it's a quibble, but I don't want my new habit strongly associated with smoking analogs. There is no smoke. There is no fire. Also, the term has many strong points:
Personal: It's mine. No homeless dude, I only have one, and I am *NOT* going to give it to you.
Nicotine: I'll cop right to it... I'm addicted to nicotine. Hey, at least it's not crack, right?
Inhaler: Explains the "fog" that I inhale, and if the person is a smoker, they're pretty likely to guess the reasons behind that and "get it". If not, the term is neutral enough to warrant follow up questions. To a non-smoker, this sounds rather clinical and doesn't seem to draw as much ire as something more closely related to smoking.
I'm going to skate the thin edge of the "no drug talk" rule here, so please let me know if I'm out of bounds on this.... I don't like to use the term "vaporizer" if I can avoid it. Yes, it's an accurate term that describes the device... BUT there are also vaporizer devices that are not used for nicotine. With the exception of a "nebulizer", most of the people I know and run into immediately think of a "vaporizer" as something used to consume illicit substances. I sidestep this issue by calling it an inhaler.
Interestingly enough, one of the first times I used this phrase, I was unknowingly speaking to a doctor. He said "Inhaler? What was it prescribed for?" I told him it was not a medical device prescribed to me, but after using it for three days I had reduced my tobacco consumption by 95% (at the time) and could easily see myself stopping the use of tobacco altogether once I was completely used to the device. He seemed very impressed.
-rick