A few weeks back I thought of using a mountain bike to commute when the winter comes. So I got my son's mountain bike road worthy and took it for a test ride. Boy, did I ever get tired!!! And it's so slow.

The gear ratio of the 15th gear is 48:14. My Sekini 10 speed's 10th gear is 52:14. The extra 1" diameter on the wheels also helps.

So in my opinion a mountain bike is just too slow for commuting. Even if you plan to use mostly park trails, you'll have to come out to join traffic at some point of your trip.