I try to keep riding outside as much as possible in the winter. I will not ride in icy conditions, it's just not worth the risk to me as I already have one hip replacement from a bike crash. I find that I feel like I'm working harder but still going slower based on power numbers and heart rate. It's also more difficult for me to maintain a good cadence, maybe it's the tights or just the viscosity of the blood I drew the line yesterday when it was 11 deg with 15 mph wind when I checked the weather. I live right on the coast and a lot of my riding is totally exposed to the wind so that has a major impact.
I'll hit the trainer for intervals and threshold work a couple of times a week to maintain some high end fitness and do endurance rides outside.
So far I've been able to maintain my mileage but December was quite mild. It seems like reality is finally raising it's ugly head so we shall see.