I've never, ever used any kind of pannier, saddlebag, or any other bike-mounted container. But since I'm happy with my backpack, I probably won't bother trying anything else out.

What I can say is that I replaced my cheap one with a bit more expensive backpack, and it was well worth it. With extra padding, an aluminum stay down the inside, and a waterproof rain cover, it fits nicely and works in all weather.