I picked up a Nikon 500mm f/8 Mirror lens locally for $80, and finally found time to mount it on my 1D MK IV today. I am really challenged when it comes to manual focusing, my old eyes are not that sharp. I had to fool with the exposure, and when I finally got it right, I put the camera into live view so I could use the magnifer to focus, and the exposure was 3 stops off.
For those who don't know, this breed of mirror lens has a fixed f/8 aperture, and are manual focus. As long as a adapter exists, they work equally well (or equally poorly) on all bodies.
After getting exposure set once again, I was able to get a couple of in-focus shots. Its pretty frustrating to use.
As expected, its reasonably sharp, and for a 500mm lens that only cost $80, its a steal. The sharpness is not going to blow you away, the birds were near MFD, so the depth of field is a bit shallow. The awful bokeh was a given, but those out of focus highlights were worse than I expected, when compared to a Sigma 600mm mirror lens that I had a few years back.
Still, for those who can't afford a real 500mm lens, a mirror lens is a cheap alternative. I should go out and try my 100-400mm l on the same basic shots and crop, but I know it would win. It cost 20 times as much, so it should be better!