I've used one of these for years and recommend it. It's a bit of a sleeper, because it's not an L, but anyone who thinks its less of a lens for that reason is looking more at the red stripe than at the images it produces.
These are what L lenses were before the L designation. didn't these have flourite elements in them? I know the later FD models just before the introduction of the L series had flourite elements.
-> David, thanks for the good word.
-> j-dogg, From what I have read, there was a green line for lenses with fluorite elements. The red line 'L' are lenses that can have fluorite, UD or aspherical elements. The 600mm doesn't have any of those, but still offers a good level of image quality.
-> gmikol, replying to your PM