We have been using Spinfire for a good while to communicate with those folks who have no (or limited) 3D capabilities. We're only using it to convey concept and overall product structure, not for CAM or anything requiring reuse of the 3D data. Its a handy enhancement to .sat files with one of my primary vendors, as they can section complex assemblies and so forth. The other nice attribute is that the viewer is free and can be distributed to our sales reps and customers. This allows me to communicate well without forcing our recipients to fork out bucks for software. We did an entire design review with a client over the phone while both running the viewer.
We export as 3D studio files with units set to millimeters. This seems to give a good visual representation to the customer.
As soon as a free or low cost IronCad viewer is available, we would likely switch.