I have a 60km RT commute and use a backpack. It's convenient and relatively aero which is important for longer distances. I think it weighs 5-15lbs depending on what I'm carrying. I lug around some stuff (toolkit, spare lights etc) that I rarely use and use it mostly for food and clothes.

I don't get a sore back but I do get a sweaty one. I sometimes bring a few days worth of lunches and then leave the pack at work. I still get sweaty so I'm not sure it makes much of a difference.

I have considered getting a rack and strapping the pack to the back. My only concern is I have several sections where there are some sharp bumps on the path. I normally bunny hop over these and not sure how well that would work with an extra 10lbs on the bike. I don't take my computer often but think it would be better off on my back.