The first winter with the Mustang I drove the stock Pirelli's. In light snow and careful driving, you're ok.
If you get several inches of snow (like I do in Michigan)...you'll wanna park the car if the roads are not clear. I tested this myself 2x and got stuck 2x.
This year I bought Michelin X-Ice Xi2 snow tires and they are on the car right now. We are "supposed" to get some snow this weekend, so I'm glad they're on the car.
My feedback so far on this tire is:
They work well in the dry. Handling is not as crisp, but you're gonna get that with a snow tire. In the wet, they can slip in 2nd gear if you're ON the gas.
That's all I got for now. If you drive in any decent amount of snow, do yourself a favor and save your car.....get snow tires.