Bought a pair of these last winter and was a little sceptical at first! Luckily they turned out to do the job really well. I use them for winter training rides with neoprene over-shoes, and where before my feet would get quite cold in severe conditions, these sock keep your feet warm in all but the really cold weather.
These are also great for mountain biking where I don't have shoe covers for my MTB shoes. As well as doing a good job of keeping your feet warm, they also help to keep most of the water out!
The only downside is that you have to be careful when you wash them. They don't seem to like being machine washed, and don't even think of tumble drying them! - the membrane sticks together inside!