In this book, Engels (with the assistance of Marx, of course, although the latter died before thoroughly discussing the issue at hand) established the philosophical basis for the socialist philosophy. This act was in essence aided by the refutation of Feurbach, a preceding philosopher who managed to set some cornerstones while demolishing others, particularly with his 'idealistic' understanding of religion as 'a universal form of love' (hihihi). Engels excused the latter due to the indirectly enforced state of isolation he was living in, noticed his excellence, and moved on from that to demonstrate the philosophical framework of socialism, manifest in its most excellent of forms as the Marxist perception of history, eventually leading to communism.
Of course, this work came later on, after the death of Marx; thus, and due to the heavy experience of many years, one manages to sense its philosophical maturity and comprehensiveness, an aspect which might have been missing earlier on. The center of that ideology is still the same: studying history as a conflict between the two economical classes, a materialistic approach at its best. The rather 'abstract' ideas of Hegel, particularly his well-known dialectical process, has been borrowed and applied on history; Hegel employs his method on a rather notional realm of ideas, and not in a practical sense.
I am not in a position to accept nor refute such a philosophy, although I disagree with it on a fundamental level. From my perspective, it is rather simplistic to limit the study of history and narrow it down into a polar opposition, although the latter probably existed (and still does); to be fair, nonetheless, this struggle during the times of Marx and Engels was evinced the most, and that is mainly due to the industrial revolution; one would have to be blind to dismiss this observation. However, this does not mean that it is safe to project that on the entirety of history, observing this seemingly infinitely complex field of study into a mere opposition between the two classes. That is to say the least, I still have not ventured on dismissing communism as a solution for that (something which requires more study; I might agree with it after all, but this is highly unlikely; I am, eventually, an abstract idealist - my realm is elsewhere).
I admire both Marx and Engels, I think they were both exceptional philosophers who managed to give life to the Hegelian work, as they were true to their cause, and consistent with their currents of thought, a virtue which marks excellent minds: consistency. I also firmly believe that most of their followers have not read their books.
(Here, I would like to recall that the eventual aim of an educated mind is to entertain thoughts without necessarily accepting them.)