Air Cargo Carriers are the largest operator in the world with around 30 in service. They fly mainly for UPS out of Louisville to various weird and wonderful points across the Midwest and South-East US.
I was lucky enough to fly the full-motion simulator (the only one in the world I think) at La Guardia back in 2007.
My company at the time built them a fixed base device which is still in operation at their Milwaukee base. Here is a video of it in operation:
http://youtu.be/5aPZgAA0Cdc