I thoroughly enjoyed the book, I think it can appeal to historians and academics as well as readers who are not normally that much into history. There are references and bibliography and the linguistic and political neutrality of academics, but at the same time the story is accessible, engaging, narrated in good structure and unpretentious language.
Personally, I prefer economic and social histories more than political or institutional ones, and this book is mainly about the latter, but that makes sense considering the top down, authoritarian regimes described. Perhaps it's a small weakness that the economy parts are a bit sparse and also that the author's speciality shows when he talks about his topic (the USSR), the parts on other communist countries go a little bit too quickly. There is some repetition (especially in conclusions, summaries and in the last chapter), perhaps it's not necessary as the history is very well written and allows the reader to make up his own mind. Details like the above are probably inevitable in such an ambitious, wide-ranging book, I couldn't put them down as faults.
About the usual question (in such books) of perceived bias, I can't imagine many people being put off, the author casts a critical eye on communism and explains why elements of totalitarianism/authoritarianism go together with communism. But even if the reader disagrees, the point is not unpleasantly forced, the book focuses on story, not rants or polemics. There's no obvious right wing bias either, the occasional successes of communism are also explained and communism is contrasted to democratic socialism and people on the left like Orwell or Bevan who I thought are portrayed in a flattering light.