I don't like panniers or on-the-bike rack for anything other than full-blown touring because I would rather carry the weight on my back or hips, so I have several different backpacks that I use instead. This gives me the freedom to choose more widely when I am selecting a commuter bike, but recently I have begun to think about building a dedicated bike to use for grocery or hardware store runs. Until then however, I will continue to use one of the three bikes that put the biggest smile on my face when I ride them, or suit the conditions the best:
My primary commuter/transportation bike is a Serrota T-Max steel mountain bike frame with a full collection of carbon fiber and XTR parts (just 'cause they look so cool, work so well, and they give the bike character). The frame, derailleurs, shifters, and chainrings are just about the only parts that aren't carbon fiber, so it weighs in at just over 20lbs. with the Spinergy Rev-X-Roks bladed wheels, and just under 20lbs. with it's other set of Spinergy Xyclone spoked wheels (even with the steel frame and lights!). It rolls pretty quickly on its Maxxis 310g tires (not much use off-road, but great for road use), handles very lively, and it gets me where ever I need to go with a big smile on my face everytime.
My Fixie usually gets used for short runs or dedicate training rides within a ten mile range, but I have done a few fifty mile plus rides on it and it never fails to cleanse my head of complicated thoughts. If you haven't ridden a fixed gear bike you should try it sometime. It really returns you to the roots of what cycling was about a hundred years ago, and there is definately a "spiritual" aspect that you will not get on a more modern configuration. Basically its just a Pake frameset with a collection of eBay-aquired parts, but it get borrowed more than any of my other bikes....
Lastly is my Opie, which usually only gets used for commuting during the winter, but is used as a freeride/urban assault bike year round. It has two extra wheelsets, each with Nokian studded snow tires (one set of Extreme 294s and one set of Hakkapeliitta W106s) and when no one else will drive to work, I get there. It's really just a mix of otherwise unused mountain bike parts that I didn't want to just throw out, but it just takes any abuse that I dish out to it and keep rolling.
Come to think of it, I really don't need a dedicated "commuter" bike. If I can't carry enough groceries in my backpack during one run, I get to do two.....
