I’ve been very surprised at how good the Canon 28-70 f3.5-4.5 Mk2 is. I bought this on ebay as part of my - Do I want to change from Nikon to Canon? - set of cheap lenses and It has been a real surprise package. It doesn’t feel as nice to use as the newer USM lenses but the results are excellent. Manual focus feels sort of scratchy and the zoom ring gets stiffer at the wide end, but it’s the results that really matter. If you’re looking for a cheap Canon standard zoom with no compatibility worries - this is it.
Sharpness in the center of the frame at 28mm is very good even at full aperture - corners are slightly soft but from 35mm to 70mm almost the whole frame is very sharp and excellent from 50mm to 70mm - at full aperture. Only the extreme corners are slightly soft.
Results at f5.6 are even better. At 50mm - 70mm results are excellent in the center and this extends over about 90% of the frame - almost right to the extreme corners. 28mm performance isn’t quite as good as the 50-70mm results but only by a very small margin. Really from f5.6 on results from this slightly worn old lens are excellent by any standard.