I have two commuters - one is a steel frame mountain bike. What's nice about it is I can take "shortcuts" (dirt roads, paths) that I can't on a road bike. It can handle the klunk. And I sit upright, which helps people see me better (or I feel like they do.) I go as far as 15 miles on this bike and then after that it gets uncomfortable.

When I am "training" for a cycling event, I take my road bike on my commute and pack stuff (clothes, food) in the day before, and then simply wear a small fanny pack for my necessities.