I wore this boot ice fishing in Northern Minnesota for 7 days last week (middle January) and they kept my feet warm.
For about the same money, however, I could have had a boot rated to -100 vs -40 from Cabela's.

Updated March 2018: Bought this pair in 2010, wore them approximately a dozen times but then the seems came apart suddenly, not gradually. Turns out, the good old Canadian Sorel has gone to Viet Nam. Don't waste your money.