For the horses who live out, we rotate the fields all year round, so there's no "winter fields" as such. There's hardly been any rain yet so all the ground is holding up well at the moment. All the big fields are on hills anyway so it tends to just be the lower areas, and where the hay gets put out in winter, that get totally mushed up.
The paddocks on the yard, for the stabled horses, are currently wet and muddy by the gates but otherwise holding up fine. Not much grass left though! We use the paddocks all year round and turn out regardless of how wet they are, my boss refuses to deprive stabled horses of turnout just because of mud. They recover fine come the spring.