Depends on which ones you get. My buddy got a cheap pair (no excuse as he has plenty of money). They had a plastic shield on the back.

On our ride on the Selkirk Loop, a rivet let go and the plastic backing got caught in his rear wheel. He flipped over his handlebars. He punctured his abdomen along with a bunch of other cuts and scratches. So what does he do when he gets home? Yep he called Nashbar got them to send a new set of bags of the same type.

You get what you pay for.