In more than one way it will be the killer application, for not only will it be the application that makes everyone want one but also the application that will send a very, very powerful enemy onto 3D Printing: Printing car replacement parts.
Think of all those small and not so small little bits of plastic that you have to pay for through the nose when (or should I say, as soon as) they crack. On your car, your motorcycle, your various other appliances. But with car manufacturers this will not be very popular, as one can imagine. They make quite a bit of money with spare parts and the planned obsolescence of various parts that "just so happen" to wear and tear.
Now, with patents it's fairly easy to keep other manufacturer from ruining this very profitable market. It's way harder if every Joe Random can make their own parts, essentially for free.
If you thought the battle of content owners vs. file sharing was fierce, just think what the battle between car part manufacturers vs. part printers will be.