Nobody knows what the long-term effects might be, but if there are any, they are likely to be on a minuscule scale compared to the effects of tobacco smoking. You're going to be inhaling the same stuff they put in asthma inhalers and post-op lung transplant patients' nebulizers, plus cake flavorings. The nicotine can be virtually ignored unless you are pregnant or have severe heart disease.
Some things that can be stated here:
1. Ideally you wouldn't inhale anything at all for 40 years (except clean country air). But if you did, you have to expect that some people will be intolerant to it. Some may die, due to previously-existing lung disease or an intolerance to one of the ingredients.
2. The death rate from e-cigarette use will most likely be so incredibly small compared to that from tobacco smoking that, for all practical purposes, it may be possible to consider it of no practical consequence. But - we don't know yet.
3. After worldwide use by millions of people for several years, no health incidents have been reported. There are few materials that could have that same profile except maybe pepsi and apples. Even aspirin would fail that test. Also, there are no reports of children dying from drinking a bottle of refill liquid, either - probably because kids don't like to drink stuff that tastes like drain cleaner.
It's safe, but not absolutely safe. It will harm some people but they are likely to be an unlucky, vulnerable few - few being the operative word, most likely. Over 400,000 die from smoking every year in the US, and if all smokers converted to e-cigs the death rate would probably be a tiny, tiny fraction of that. But to be honest, at this moment in time, we don't know the full story. It's not likely to be as safe as genuine Swedish Snus for example, where 25 years of research shows them to be safe, as long-term inhalation of materials is likely to have implications for some.
No one is going to be harmed by 4 or 5 years of use, as there has been a global 'trial' that clearly tells us this. However if you are an ex-smoker with emphysema, COPD and heart disease, then there may be implications and you might be better off going with Snus in order to maximise your remaining time. People with emphysema have a higher risk of pneumonia, so inhaling anything is probably best avoided.