Spending my first winter with the 535i xDrive. Coming from an Audi Quattro Allroad, I have to say I am impressed by BMW's xDrive system. My fear of not having dedicated winter wheels were unfounded.
The same set of all-season tires(from the Summer) performed as well as I could hope, cornering and tracking were rock solid- no fishtailing at reasonable speed. I am sure having dedicated winter tires would be even better, but having driven for 20 years in Minnesota, I can confidently say Xdrive+good all-season tires are plenty good.
