Beware, anecdotal evidence ahead:

I use an e-cigarette, but I have never smoked cigarettes habitually (I've had about three in my lifetime). I do have a cigar once every six months or so. About once a week, I use my e-cigarette like I would use caffeine while gaming on the weekends. When I am using it for its stimulant effects, I prefer a high nicotine concentration. Other times, I use nicotine-free liquid because I enjoy the vapor and have a terrible habit of putting everything in my mouth. If I don't use the nicotine-free e-cigarette, I start eating paper, chewing on pens, biting my nails, or chewing my hair. It's embarrassing, and the e-cigarette has been the only workable solution.

I did go through a period of prolonged use, at a high nicotine concentration. The withdrawal was not fun. Now, if I find that I'm using it more than I like, I again take the same approach as with caffeine. I take a break of about two weeks, and that seems to help.

passive_fist's source seems to be legitimate, so I wouldn't recommend prolonged, habitual use. If you find that you enjoy the flavored vapor without nicotine, then you might consider that route.