I've had great success with Sorel-style boots, specifically Baffin Bison boots. I use them in combination with large Freeride/Downhill-style studded platform pedals such like the Blackspire Flatfoot. When it gets really cold and/or during long rides, I add chemical toe warmers.
They take a second to put on or to remove. They're high enough that they block snow from coming in, making gaiters obsolete - they catch your pants so you don't have to tie them otherwise. Oh, and they're very comfortable for walking. They're also somewhat inexpensive and rated down to -40C.
However, in the fall and spring, when the roads are clear enough that it is safe to use SPD again, I quite like my Exustar SM-450 SPD boots. Of course, they're somewhat of a pain to put on and remove and they're not very walkable. Also, I find it necessary to use gaiters or some other means of tying my pants so they don't get caught in the chain or something bad like that.
During the winter, even though I have a pair of studded tires, the road conditions get so bad that I sometimes have to put a foot on the ground. I wouldn't want to be tied to the pedals when that happens, which is why I must sadly abandon SPD until pavement is no longer covered by inches-thick patches of hard ice.