At my dad's work (Rocketdyne-owned by Pratt&Whitney, used to be owned by Boeing, makes engines for space craft), they have several large rapid prototypers that are used to create prototypes for different designs to test, and they even make an air duct used in F/A 18s.
They're really fun, but what they can create is rather limited from my understanding. They can do very intricate designs (even objects within objects, like a triangle INSIDE of a complete sphere), but only out of certain plastics, and only one material at a time, so the usefulness is limited. You couldn't build an engine block with it, because it can't work with the metals.
There was an article in Make Magazine about the RepRap and it works with metals which have a low melting point, such as Field's metal ( http://en.wikipedia.org/wiki/Field's_metal ). This allows the RepRap to include wiring. Additionally, the machine can pause to allow electronic components to be inserted.
They're really fun, but what they can create is rather limited from my understanding. They can do very intricate designs (even objects within objects, like a triangle INSIDE of a complete sphere), but only out of certain plastics, and only one material at a time, so the usefulness is limited. You couldn't build an engine block with it, because it can't work with the metals.