I've used non-OEM batteries in my g6, XTi, and 50d without issue. But, I am reluctant to do the same in my 5diii. First of all, I get way more shots per charge than the older bodies achieved. The "shots per battery" ratio is better so this reduces the amortized cost (although it is still higher than the off brand). Second of all, I am often in remote places where replacement batteries are just about impossible to obtain (OEM or non-OEM).
But in the same vein, that's also why I am reluctant to get a non-Canon lens. I don't know what the camera - lens communication and function could do in the future, but I am pretty sure Canon will see to it that their lenses work with their cameras. It's the same logic, eve if this is a "monopoly" of sorts.