+1. Also, "light" is highly over rated for a commuter.
In addition to swapping out the tires on your MTB, you can change handlebars or add some bar-ends for additional comfort at very little cost. As an added bonus, converted MTB commuters are less appealing to thieves.
Originally Posted by
a1penguin
I would try this. I see plenty of people at work commuting on mountain bikes. A five mile commute is short enough that any bike should work. Putting a pair of thinner slicks on the mountain bike will probably give you a bike with a smooth ride.