I've seen some really interesting gear work done with these and I can't imagine how much prototyping these gears any other way would have cost. There are many duifferent approaches too. The van of liquid that hardens with the U.V. beam and as the table lowers the part is formed opposed to the "glue gun" like approach. Very different approaches. There's a company in not too far from here that makes these machines. This one is $2700 dollars and uses ABS plastic wire! :
3D Printer
These are many ways that this is done. This is a technology that if you have the proper tools, and know how, you can make a machine for less money than you can buy one for.