I love my backpacks, I have a small one and slightly larger one. I carry my small laptop, brolley, notebook, charger etc and my work lunch in the larger one also.
I've heard they are better for your back than carrying heavy tote bags, which I used to always carry and felt hampered with it, especially if I'd got other shopping too.
A friend of mine has said they damage your back. She's also said they are only for hikers and lesbians! Now I feel I shouldn't be using one.