If replacing tobacco. Electronic cigarettes contain water, propylene glycol, nicotine, and tobacco scent, but don't have the smoke, tar, and other cancer-causing substances. Electronic cigarettes are safer than real ones but still contain nicotine, which is addictive and narrows blood vessels, which is bad for one's circulation. In general, it is not healthy to inhale anything other than clean air.
Better than tobacco. I wouldn't say that electronic cigarettes are "good" for your health, but electronic cigarettes do not contain the carcinogens that tobacco smoke have. So, it is definitely better for your health than smoking tobacco.