Depends. I brought my 11 home on the nicest day in December here in Minnesota. It was not a fun drive. The main roads were OK, but the residential streets were hard packed snow, so they were slick. The Brembo package on mine with the PZero tires (summer only) are WORTHLESS. It was like driving a hockey puck.
That said, I drove Mustangs plenty in the winter before this one. It all depends on the rubber you put on the car. Low profile summer only tires are exactly that, summer only. That compound is hard as a rock in 30 degree weather and offers no traction. On the other hand, a decent M+S tire will get you through just fine. Just don't drive like a nut.