I'm a 19 year bike commuter. A big issue for panniers is heel clearance so the pannier needs to have a tapered shape or a connection to the rack that allows it to be mounted high enough or far enough back to be out of the way of the heel. Also a quick but secure attachment mechanism to the rack. The ones that have open hooks at the top and a bungee cord hook at the bottom have a tendency to jump off the rack. The one I have now has hooks that go over the rack bar and a rotating wedge that comes up from below and it is much more secure but pretty easy to pop on and off. I don't like commuting with back pack since they make you top heavy. Also a big problem for back packs is back sweat. It would be great if that could be fixed - maybe with a egg carton type panel against your back that allowed air flow.