I just sprung for a pair of Shimano MW81 winter boots and rode them for the first time last night. 1.5 hours of icy/wet single track riding and temp was about 20 degrees at the end of the ride. Worst case scenario, I put my foot down ankle deep into a bog and later fell full on into an icy mud puddle. I finshed the ride cold as hell but the shoes were total champs. My feet were downright toasty.

I bought the shoes a little big (I usually wear 43 or 43.5, I bought 44s), and wore medium thickness wool socks. My feet liked having room inside the shoe. If they had been any warmer they would have been too warm.