I have been commuting to work (about ~8 miles each way) this summer about 4 days a week. I started off packing all of my stuff (work clothes, food and documents) into a big Chrome somethingorother bag. I only did that for a few weeks as I often had to fiddle with the straps depending on how much I was carrying that day. Also, I found that it obstructed the view over my left shoulder when it was fully packed. Looking back into traffic is paramount to me so I stopped using the Chrome bag and started using my Dakine heli pro backpack. This bag feels much more in place and less likely to transfer weight around on your back (it has a chest strap and waist strap). I don't have any visibility issues either and the padded back prevents my lunch or any other oddly shaped items from poking my back the entire ride. The only downside being that in the summer when its super hot it does make me sweat a lot more as it covers almost all of my back.