For me the 70-300L has proven itself worthwhile. It's perfectly sharp, IS is four stops, you even get solid background blur and then it's light and much agile than the alternatives. If I need reach I'll put it on a cheap 60D.
No way, it doesn't outresolve either the ff or crop sensors - I'd say it's absolutely "sharp enough", but no comparison to for example the 100L or afaik other very sharp zooms like the 24-70ii or the 70-200is2.