I find the 500 great. Although I chose it over the 600 mostly because of the price, and have to say if I could afford to, I'd probably upgrade, I have no complaints over image quality, ergonomics, or ruggedness. My walkaround combination since the f/8 autofocus update to the 5DIII is the 500+2x extender, which produces good images when stopped down to f/10, at least in good light. I'm not strong, but I can handhold that for a few hours. When it gets darker, I swap to the 1.4x extender, then the bare lens, but most birds are too skittish to allow you to get close enough for such a short focal length. I also find stacking the extenders can produce usable images if I can't get closer, although this is manual focus only and works best with support.
I'm sure for larger animals the shorter lenses would be more appropriate, but the 'never enough focal length' mantra for birds really is true in most situations.