I took the Canon USM IS 70-300mm out today to try to zero in in what it can and cannot do.
It is hopeless at 300mm. "Soft" does not describe how bad the focus no matter what the shutter speed or f stop. (My sainted 100-300, RIP, was considerably better.) So, I'll just not try to push it that far.
Having said that, from 70-200mm it is pretty darn good. From 200-250mm it is OK. All things things considered, pretty good performance.
Here is a shot at 160mm, f5.6, 1/320 ISO 800 with a polarizer. Pretty good. The low speed of the lens is apparent as can be seen in the high ISO used to get the shutter speed up to acceptable level.
As Dirty Harry might have said, "A man needs to know his limitations."