First we need to define what "E85" is.  E85 is a biofuel mixture which contains 85% ethanol alcohol and 15% gasoline. It is available in most of the United States, however in some states distribution is sparse.  E85 is more popular in southern states which are closer to the companies that are making the ethanol.

As you might suspect, a Flex-fuel Vehicle (FFV) is a vehicle that is designed to operate E85 fuel. In fact, they will work with gasoline, E85 or any combination of the two. You could think of FFVs as "transitional vehicles" and E85 as a "transitional fuel". They are engineered to bridge the gap between our current predominately petroleum-based (gasoline) supply and steadily growing bio (plant-based alcohol) fuel supply.

Not all vehicles are FFV, in fact most aren't.  In the US, the largest proponent of flex-fuel vehicles has been General Motors. Many automotive manufacturers just haven't signed on because there isn't a lot E85 stations in the US yet.  It's a bit of a chicken or egg kind of thing.  In other countries, E85 is vastly more popular.  In Brazil, for example, most gas stations carry E85 fuel because the biofuel is made locally and in huge quantities.

Does E85 save you money?  Well, currently it doesn't.  While the prices of E85 are typically less than standard gasoline, there is less energy per unit volume (27% less) which means less fuel mileage.  This, of course, would change if the prices of E85 would decrease, which is always possible if supply increased.

And what about the future of E85? The fuel distribution infrastructure for ethanol is still in its infancy, and it will take many years and many millions of dollars to fully develop. 

During the transition from petrofuel (gasoline) to biofuel, vehicles will need to be able to run on both types of fuel as availability of one or the other may be spotty. E85 fills that immediate need by blending the two fuels together.