It is incredibly sharp, but the bokeh is not that good. Hard to describe - kind of a "nervous" background. Of course this is being nitpicky and for the price there is nothing comparable that is that long and that sharp. But after buying one from Canon refurb about three years ago and using it for maybe a year, I sold it. Most users seem to love it, so maybe I am just being overly critical. What I really wish is the Nikon 500 5.6 PF came in a Canon mount!
The other issue with this lens is the autofocus was not nearly as good as my 70-200 2.8 . It would fail to lock with dark subjects or have difficulty shooting past fences at zoos (even with focus limiter on) when my 70-200 would not hesitate.