I commuted on a mountain bike for a while. It was okay, I had a lot of fun jumping off curbs and such. Used Kenda SmallBlock 8s which were really fantastic on the terrain of my local mountain, and okay on the road. But ultimately I wanted to ride farther and faster and carry more stuff, so I got into road bikes.

When I did, especially when I got my current bike, I realized what a tremendous difference there was. I encourage everyone who uses a mountain bike to consider getting a nice, tough road bike and trying it out for a while.