I have an Arkel combination panier and backpack.

The backpack straps can be folded and hidden with a zipper cover. So you can take the bag with you quite comfortably. Traditional panniers have this odd rhomboid shape and not a very comfortable way to carry when they are off the bike.

The attachment to the bike rear rack has cams that secure the hooks from the bottom so they squeeze the rack frame.

The hook at the bottom of the bag is free to relocate laterally to secure the bag further to the back of the bike so that it won't interfere with your heels while pedalling. This avoids the odd shape that tradition panniers have.