I am working on a proper review for a journal, so I can only say so much, but I believe that Werner's book is a singular contribution to critical theory.
I think of Open Marxism as a tendency broadly grouped around a composite of works produced by Richard Gunn, Kosmas Psychopedis, Simon Clark, John Holloway, and Werner Bonefeld, first among a host of critical intellectuals they have influenced (e.g. Adrian Wilding and myself), and themselves the inheritors of Frankfurt School critical theory via Hans-Georg Backhaus, Helmut Reichelt, and Johannes Agnoli.
At the same time, there are other appropriations of critical theory that come through Gillian Rose and Moishe Postone, respectively, and they have existed at a certain distance from Open Marxism. And yet there have been connections. Adrian Wilding has in his work brought together Open Marxism and Gillian Rose, as Bonefeld's book brings Postone and Open Marxism together. My own work has been deeply influenced by these strands and has been driven by a reconciliation of these various elements (see several of the earlier pieces on this site.)
If you have a commitment to a truly critical Marxian analysis or you want to engage with a Marxian perspective worthy of intellectual engagement (hostile or not, as for example my compatriot at Insipidities might be), this book should be a key reference point.