I have been thinking lately of switching from backpack to panniers, for the comfort, but I still like the original reason I went with a backpack. A backpack allows me to commute on any bike because not all of them have racks, or can take racks. Also, I sometimes stop along the way (grocery store, bank) and the backpack is easier to take with me because it already is with me. I probably will stick with the backpack because, after analyzing all the factors related to me personally, the backpack is the best fit. Not perfect, just the best of all the alternatives.

For the last 4 years i've been commuting with a backpack for short commutes (4 miles round trip).

I just moved and now my commute is 10 miles round trip. At the end of the day my back has started to kill me and it's just too hot in Texas in the summer.

Today was the first day I used my backpack bungie-corded to the top of my rack. The ride was hugely more enjoyable, I felt great at the end of the day and felt I could ride another 20 miles if I wanted to.

I'm definitely getting panniers, just have to figure out which one!

Bikes turn by rotating about their center of mass; to optimize quick handling, a bicycle must centralize its mass. The rider is by far the most massive thing on the bike, and it's pretty easy to see that the center of mass of a bike with a rider is relatively close to the rider's center of mass. Adding mass closely to the rider's center of mass will yield both the quickest rotation because it will result in a smaller radius of gyration of the system as it rotates around the CM. It will also cause the least impact on the handling of the bike because it will require about the same amount of effort and the same degree of movement to move the wheels out from under the CM, where steering geometry causes the front end to pivot to track the corner and camber thrust steering makes a not-insignificant contribution.

Also, bunny hops are way easier with 10 lbs of crap on your back than with 10 lbs of crap over your rear axle. Standing up and putting the hammer down to make a light is really lame on a trunk bagged bike, also.

I think he's trying to say that bikes handle better with the weight on the rider.