I use a backpack for my short 4ish mile commute. I use a Mountainsmith Tour lumbar pack when I don't need to carry much, and a Banjo Brothers commuting pack when I have to carry a bit more. For my short commute (to a company that does not have showers, storage for clothes or food) the backpack works better for me. For my previous longer commutes, I either pre-staged and carried little, or used panniers.
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"Let us hope our weapons are never needed --but do not forget what the common people knew when they demanded the Bill of Rights: An armed citizenry is the first defense, the best defense, and the final defense against tyranny. If guns are outlawed, only the government will have guns. Only the police, the secret police, the military, the hired servants of our rulers. Only the government -- and a few outlaws. I intend to be among the outlaws" - Edward Abbey