I ride the road bike on all but the worst weather days. Had the MTB out every day this week due to snow and ice, and I've realized I quite like it! It's better feeling on my neck, shoulders, arms and back, but is heavier and not as fast/efficient. That said, I can do the legs on my 30mi RT trip at least a couple of minutes faster on my road bike.

My road bike is light and alu, so I feel EVERY crack on the road of which there are many. I'm seriously contemplating putting together a rigid MTB commuter with some narrow commuter tires (Marathon Plus, for instance) and a rack and fenders.

Love my roadie, love to spin and go fast, but the winter is bad enough it's impassible on a roadie.