I've never wanted to spend the money on a real winter shoe - so on the road I will wear over-booties, toe covers and the like. I don't have a problem clipping in with those on the road, but the few times that I have worn overbooties on the mountain bike, I have trashed them and torn them up and made a mess of them so bad that by the time I'm done with them I can't unclip cause there are threads falling off and getting in the way.

I have a pair of mountain bike shoes that I can put heavy socks in i.e. wet gore tex ones for really wet riding and when it is not wet, I wear a liner sock and a woolie boolie type sock. This works okay, but my feet do get a little cold!