The Sealskinz thermal ones are nice if you want one soft, warm, waterproof layer. However, I took to using the Gore oversocks during the worst of the winter, over whatever weight/warmth of sock I thought was most appropriate. The advantage of the Gore ones is that they are less susceptible to inappropriate laundering. I find that the Sealskinz eventually shrink in the foot and go baggy around the calf – encouraging the ingress of water.
As always, the biggest issue is water running into them from the top. Wearing longs over them helps. Alternatively, make gaiters from the wrist of a pair of household rubber gloves.