I find it pretty good, wouldn't rate it especially highly at f4, but stop down and it works pretty well.
what it is... is a nice walk-around focal length on 645
80mm was a little long on 645, (meant for 6x6) then add a crop factor 1.1 and its even longer.
the best wides for contax 645 are using Schneider Curtagon 60mm lens (Exakta 66 fit, with adapter) much higher resolution lens at wide apertures, but of course no AF, no auto stop down, etc. cheapish though if you can find one.