In a non-bike context I'm a big backpack guy. I've had this convertible backpack from 2 Wheel Gear for a year or two and like it. For touring or the occasional big haul (my grocery store is a few blocks away so I always go on foot) I use panniers, but otherwise this is usually my bike bag.

Not cheap, but it does what I need it to do. There are obviously backpacks that do a better job of being a backpack and panniers that do a better job of being a pannier, but it's solid at both functions and very easy to convert (60ish seconds). If the price isn't a deterrant, I recommend it.