They keep you "partly" warm, but it's still uncomfortable. The problem is when riding, the wind will go thru the fabric and you'll have next to zero benefit from the insulation. Insulation works by trapping a layer of dead air (still, not moving) around you. If the wind takes that air away, you feel colder. That's why it's colder on a windy day. And on a bike, it's always windy.

Try searching this forum as well as the winter and commuting forums for "waterproof socks".