I think mountain bikes are great commuters. I use one as my sole transportation and I am on it 2 or 3 hours a day with no problem. I used to be pretty slow but I can book on it now. I have street semi-slicks (1.5 inch), but knobbies aren't that bad.
I also have a road bike, but I just like the smaller, nimble mountain bike better for the vagaries of urban riding. If you are in the wide open suburbs, you might do better on the road bike, once you get used to it. Could you try both and let us know the pros and cons?