I'm a 20 year + smoker - hate the things. Vaping has already helped me cut down, and maybe one day give up. Regarding the science of vaping, the only question I have is too do with the act of burning the chemicals in e-cigs. I honestly don't know enough about chemistry to know for sure how safe they are. But I know analogs are dangerous, yet legal. I have no doubt this whole debate comes down to money: taxation, tobacco companies protecting their profits, political corruption, the norm really lol I think the one thing that can save vaping is scientific analysis and a mature enthusiast culture to educate the masses.
Let me try to answer and ease your mind RE the science behind vaping
they are as safe as breathing, actually safer.
The main ingredient is pg or vg (glycerols), pg is the most common used and you will find it in everyday items including asthma inhalers.
http://wiki.planetofthevapes.co.uk/wiki/E-liquid
The second highest ingredient in most liquids is the flavoring itself, this is probably the most complex chemical in the mix as each flavouring is made up in a different process which affects taste and quality, you have some natural flavourings too.
The final ingredient is the nicotine. Nicotine does not react well with heat and under current levels of understanding alot of the nicotine is destroyed in vaping, as much as 50% in the heating(the nicotine is literally destroyed/rendered inert) process, nicotine is then mostly abosrbed through the cheeks and throat, hardly any is absorbed to the body through the lungs, the current estimated figure is approximately 30% of the nicotine content in the juice you use is abosrbed, the rest exhaled or rendered inert.
The carcinogenics that were discovered in a research project conducted in america are trace amounts and actually much lower than found in the everyday air we breath in large towns and cities being produced by cars etc
The last concerning point that is sometimes raised is of solder contamination. This was from a very early study on old style cartomisers. I dont believe that style is produced anymore, if it is they will be very very cheap, cheap enough to think "if its too good to be true it probably is". Again though that was trace amounts spun from the search for a negative when any negative result was needed.
Peruse the news section and health sectins for more information