It is an excellent question asked time and time again.   If you ask anyone using electronic cigarettes they will tell you how they feel as a result of switching from cigarettes to e-cigs.  However because e-cigs are such a relatively new product no long term research exists yet.   What we do know is if you compare cigarette smoke to ecig vapor there is a huge difference in the amount of harmful toxins being inhaled.   Electronic cigarettes do not contain the loads of cancer causing carcinogens and and tar found in burnt tobacco.   For most people, ecigs work great as replacements for tobacco cigarettes.  

You basically are cutting out the harmful smoke that cigarettes produce and eliminate secondhand smoke while being better for the environment and the people around you, in general.