I used to ride with my backpack until I started having pain in my shoulders and back, along with sweaty back, after a 10-mile ride. I have since switched to a touring bike with bar ends and I bought a messenger bag. I still get a small sweaty patch on my back and moderately sore shoulder but it is better. I find a rack and panniers to be the least uncomfortable, but they make the bike less responsive in an urban environment and add quite a bit of weight. I still use a rack if I have to fetch anything heavy. Books and a laptop are heavy items that I don't want on my back. I use the messenger back just for my clothes, cellphone, headphones and mp3 player. A benefit of the backpack or messenger bag is back and shoulder protection is the event of a crash. Just the other day I crashed hard at high speed, flew over the handlebar and landed on my shoulder and back. Lucky for me, the backpack took most of the impact (ripped to shreds) or I would've ended up with much more than a bruise on my shoulder and a cracked collarbone.