If you're riding a road bike that requires bending over the bars a bit, a pack with a rigid frame that doesn't bend as your back bends can be pretty uncomfortable. I have a camelback-style pack that has a rigid mesh back so that my pack doesn't smash against my back and make it sweaty. It works great on my mountain bike, but when I tried it on my fast road bike it was extremely uncomfortable so I don't use it there.