I have a couple of those old Sigma lenses. The APO version is even a bit better.
Also Tokina made a very fine 400mm f5.6 in all the popular mounts.
I also have a Tamron 200-400mm in Nikon mount and like it very much despite it usually being considered mediocre.
One thing to be careful about on these older lenses is they often have a bit of haze, sometimes not very noticeable until you view it just right.
Also a few scratches on the front or objective lens usually does very little harm and even that can be eliminated by running a fine black sharpie over the scratch. On the other hand, scratches on the rear element will harm the sharpness a lot.
Some of these Sigmas, the early ones, had to be rechipped to work on the later Maxxums, and the Minolta digitals and Sonys.. Sigma was doing it for free. This one seems to be out of a later batch.
Sigma made them in black, gray, white and green. The green ones are sort of putrid looking, really a horrible color, the white ones are known for flaking paint, so I was glad to find one of the gray ones. It's a nice dark charcoal.