Many years ago I used a backpack to carry books for a couple of miles. Then I got a rear rack and panniers, and gave up the backpack except for short rides on a bike without racks or a front bag. Backpacks tend to raise the center of gravity, and are tiring. However, if I am going to the store,,I sometimes carry a very light weight pack made of ripstop nylon, in which I put light bulky things like paper towels.that would otherwise take up too much room in my handlebar or saddle bags..

If I really wanted to use a backpack often, I'd consider the Osprey bags.