Whether it's comfortable or not depends a lot on the weight, and on your sitting position. I wear a very light, narrow backpack on almost all commutes, also on my road bike. Some days I'll take a larger pack so I can switch some of my clothes at work, but it's still a narrow bike-friendly pack, and then I'm usually on my mtb. Most of the time I prefer a light pack to having the extra weight of a rear rack, makes my bike feel all funny ;-)

Going climbing I'll have a heavy pack - and swearing at it all the way... I also lug groceries home on my back, but that's just up a 300 m long hill. Sitting upright and going slow you can carry a pretty heavy pack without too much trouble.