One person was courageous enough to be straight with Paul McCartney...Nigel Godrich (Radiohead's producer) who produced his album "Chaos and Creation in the Backyard." Apparently Nigel listened to Paul's demos, and said of a bunch of the songs, "These are just okay, you can do better." He gave some concrete suggestions and basically said, "Come back when you have a collection of songs that are actually really good."
Paul flipped of course, but eventually came to see Godrich was correct. And the album did turn out quite well.