Has anyone ever used them - I think both American Classic and Cole have them and possibly a few other wheel manufacturers. I'm curious about peoples' experiences, comments, ...

Or if no one has actually tried these, what is their selling point compared with other materials?

Mg is about 2/3d the weight of Al with strength that can be not too much different, depending a great deal on the alloys used and method of construction. Mg is not as corrosion resistant.
Steve

I think the AC sprint350's are a alloy of Al and Mg

The American classic mages are ceramic+epoxy coated and they have had "zero" corrsion issues this entire year. They've sold like crazy-2 pro teams are now on them. They are also smooth riding.