I commuted for several years using a backpack 25kms each way. I was riding a road bike so rack/panniers were not an option. Not that I was unhappy with the backpack. If the weight is reasonable I like a good packpack. Once the weight of my cargo gets heavy or the stuff I need to carry is bulky I'd rather have it on my bike than on my back.

BTW - I own several bikes with racks/panniers so I've got options.