You still get the hit of nicotine which is the chemical a smoker craves, so, in theory you can use them to come off cigarettes all together. They will be minus the tar and carbon monoxide which is clearly beneficial to ones health.
The purpose of them in the long run is to lessen your nicotine dependancy over a period of time, you can get various strengths of electronic cigarette and the idea is to decrease the strength over time. Hopefully to a point where you do not crave nicotine and are unlikely to take up smoking again. My concern is, because people think they are healthier using an electronic cigarette, they will use it more often than they would have smoked oringinally. Therefore their body is getting used to MORE nicotine than before. Which, if they do start up smoking again, could mean they need more cigarettes to satisfy their addiction to nicotine!