It's not bad with a normal bookbag-type backpack. I use a backpack that's about the size of a normal pannier. Only kicked the pannier off once. Since then I have stopped doing standing dismounts anyway since I read on sheldon brown that it is not so great for the bike. If I want to pick up a lot of groceries on the way home, I can fill both grocery panniers and wear the backpack. Pretty good system for commuting. Nothing beats my xtracycle pedelec for cargo capacity though