John Berger is not your mainstream art critic. He is an independent thinker and is nobody's fool. You may find his Marxist rhetoric somewhat dated and his references to bourgeois class even silly, but his style is strong, he's informed intellectual with whom you may disagree but will respect and, if you opened, will learn few things.
Berger attributes Picasso failure (assuming you know where Picasso had succeeded) to his selection of inferior subject matter. Being of Marxist's creed, Berger would prefer for Picasso to select his subjects from a set of social problems which will connect him to a 'working class', a nation, or a movement, rather than be confined to a personal expressions. He's OK with his blue-pink period of 'being a social outcast' and considers his cubist period as his best. He also finds the merit in his work of post-war years and sees his work in decline starting from fifties. His accusations are not completely groundless but are disputable. His astute criticism of cubism, its connection with natural sciences, quantum mechanics, its simultaneity of multiple views as a way or organizing information, these are the most interesting passages I enjoyed.
I like Berger's dissenting views as a stimuli for discussion. He will not bow to the overwhelming Picasso admiration and is not afraid to provides critique that alone drives our knowledge forward. I found his book interesting and useful.