I have a transit rack and panniers on my bike. I carry a laptop, change of clothes, lunch, two cup thermos for coffee and my change of shoes. I have a 14 and 1/2 mile commute each way, so I wear biking clothes for the ride and change at the office.
I suspect my panniers weigh about 12 to 15 pounds or so when loaded for work.
I tried a backpack a few times, but, like others, I do not like the hot sweaty back and soaked shirt. The rack attaches to the bike with only 4 screws, so removing it for a Sunday ride is not difficult.