One of the great things about the sheepskin (addition to looks I like) is range of comfort. Great in both winter and summer - keep you both warmer and cooler than many other options.
It's like putting on a jacket in the winter - cozy warm fur. They do not have a big detrimental on heat transfer if your seats have that option. In the summer, the layer of air / breathability is awesome - don't get baking hot (it was 95 here in the ATL yesterday) sitting in the sun and circulation keeps bottom of legs from steamy situations.
One negative on them is the amount of charge they build when sliding out of the seat in cold / dry. I maintain contact with a ground to discharge slowly or you can get quite a shock.