The fun part is you aren't allowed to ask explicitly how healthy the owner is, so it's a total gamble. You might be stuck paying rent to someone who lives until they're 100 or they could die in an "accident" tomorrow. So in addition to checking the walls for damp when you go round, buyers surreptitiously look for medication and disability aids. Presumably some people also go to the trouble of stalking the current owners to gauge how close they are to the coffin.
To make things more exciting, if your parents bought a house en viager and they leave it to you in their will (but the owner hasn't died yet), you're on the hook for the payments. If you don't keep them up, the owner can re-sell the house.
http://www.connexionfrance.com/explaining-the-viager-system-...