Goofing around with friends, taken after returning from a hike, at 500mm on a crop (800mm effective) hand-held, low light.
When I first received this lens, I tested it against my buddy's Canon 70-200 f/2.8 with a Canon 2X extender. I couldn't see any significant differences in the images. I like to keep my monopod attached to this lens. It gets heavy very fast.
Edit: With any telephoto, there is going to be some image quality loss due to the increased amount of haze and dust that goes hand-in-hand with shooting over longer distances. And, by using Live View, zoomed in to the maximum enlargement, it's apparent that there is almost always a little bit of vibration, even with the best shooting techniques.
I think a lot of people expect a telephoto lens to work like the CIA satellites in the movies, that zoom in from a thousand miles away to read a license plate.