Practically, I carry too much stuff on my work commute to use a backpack, particularly in the winter. I use two Arkel Bugs almost all of the time. Sometimes in the summer I will downgrade to a single pannier if I am not shuttling batches of clothes or food to the office. I also have Ortlieb panniers that I mainly use for traveling/touring, but have occasionally used for commuting. They are nice when I know I will be riding in monsoon-type weather. I have used backpacks at times, but I really hate them, particularly if I have much weight in them. They make me sweat, even in the winter, and I don't like having sweat soaked clothes in the winter.