This is, I think, the second version of the Sigma 400mm f5.6 APO - so it comes between the original 400mm f 5.6 - often a sort of khaki green or grey - and the later 400mm APO Macro. I’ve used all 3 versions and found them all to be good performers. Results are excellent on film or digital - even at full aperture - and build quality of the earlier versions is lightweight but perfectly adequate, while the later Sigma 400mm APO Macro is the best of the three optically - and the best made. It’s bigger and much heavier than the others.
The main worry with all older independent, especially Sigma, lenses, including this one, is compatibility. Some older lenses just won’t work with newer digital SLR cameras. This lens worked fine with my Fuji S2 and Nikon D70 camera bodies, but a later Sigma 300mm f4 APO Macro I bought didn’t focus accurately and couldn’t be rechipped - so try before you buy if possible, or give your Sigma service centre a call. I’ve added a list of my own experiences with Signas and different camera makers below.
Assuming you can find a Sigma 400mm f5.6, and it does work OK with your digital SLR camera, you won’t be disappointed. I’ve yet to find a zoom that covers 400mm and performs as well.
Nikon bodies including digital - should be OK but try to check - especially focusing accuracy.
Canon - only very recent Sigma lenses will work with any Canon digital camera body. Older lenses give error messages and lock the camera.
Minolta - I haven’t used either 5D or 7D but used lots of Sigmas with Minolta film bodies up to the 800si and had no problems.