The Mustangs are selling so fast in West Palm that the dealers can't keep them in stock. The day I went to test drive a 500 the dealer did not have a 5 speed available for sale, just the demo - and they sold that too.
During my visit, 4 new Mustangs left the lot in 40 minutes with new owners at the wheel, and there were at least a half dozen more in the showroom negotiating. I don't know how many were in the offices doing the paperwork.
A few 500s were being sold, I suspect the demand will pick up as people hear more about the car.