i change between a backpack and a set of panniers that i designef myself depending on the conditions and my ride. if i can pack light i go for the backpack with a cheststrap to pull it in tight cos it feels like the weight is more evenly distributed rather that hanging on your back wheel. But if it's hot or i have a bit of a load to carry i go for the panniers - they convert to a shoulder bag which helps at uni or going grocery shopping... it's great changing between panniers and backpack, for me it makes for a nice variety in the feel of my bike... thats just me... im rambling, bye