I was disappointed. Singer is affectionately amusing on Marx's life and I agree with him that Marx retained a (materialist-reframed) Hegelian perspective throughout his life's work. But Singer struggles with Marx's notoriously tricky concept of value (abstract human labour), buying the argument that machines can create surplus-value. In Marxist thinking, only human labour can do that, reflecting the distinction between labour and labour-power.
Once you don't 'get' Marx's theory of value, then the theory of capitalist dynamics tends to go too, and you're left with a static evaluation of Marx's specific predictions, most of which did not track capitalism's evolution throughout the twentieth century.
Singer thereby concludes by seeing Marx more as an ethical philosopher than anything else, but that's a retreat too far. I would, however, be even harder than Singer on Marx's lamentably blank-slate view of human nature, which fatally undermines any naive presentation of Communism.
Singer also wrote this book, which is on my list:
I had thought I might be the only person in the world who cared about the synthesis between Marxism and evolutionary theory, specifically human sociobiology. But I guess that along with Peter Singer I should now add David Reich to my list of good guys too, although he's a liberal-leftist, not a Marxist.