I use the Keen Winterport series boot. It's really more of an insulated shoe with an integral gaiter, but I like that it doesn't rub on my ankle joint while providing a good air seal around the lower leg and foot. I bought several sizes larger to accommodate a second pair of socks but still have a loose fit. I've been out in 0F-5F for my hour long commute, but haven't really had a chance in the last year to be out in sub-zero, it just doesn't happen too often here.