I know nothing about 16th century Europe, the debates between Christianity at the time (other than the Reformation), or Thomas Muntzer before reading this book.
Goertz does a really good job of establishing the contextual background Muntzer lived in and the material conditions of society that partially influenced him. However, Goertz convincingly argues, his revolutionary views, "omnia communia sunt" (roughly 'all property to be held in common'), and siding with the "common man" over the clergy/nobles originates in his theology - the core being that there should only be a fear of God - not "creaturely fear". Since the peasants and artisans were being held in creaturely fear by the nobles, clergy, monks, etc. Muntzer recognized the latter as the obstacle to erasing creaturely fear in humans and eventually realized the only way to get rid of this obstacle was through an uprising - hence Goertz terms Muntzer's views as a "theology of revolution". I know very little of both but to me Muntzer's theology almost seems like a proto-Liberation theology. Could be wrong here though. Another interesting thing Goertz asserts is that Muntzer thought the Kingdom of God was immanent to history - that it could be brought about through an erasure of creaturely fear, among with communal living and brotherlihood.
Goertz synthesizes the two common interpretations of Muntzer: the Marxist view and the Church-history view (the former, that he was a "figure who reflected the progressive social transformation [from feudalism to capitalism], certainly not completely, but fundamentally all the same." The latter tend to emphasize Muntzer's subjectivity, namely his pretensions of being seized by the divine spirit. As Goertz puts it: "one group orientates its interpretation towards what Muntzer wanted to think and do; and the other towards what he was obliged to think and do."]. He does this by emphasizing an interconnection between Muntzer's theory and practice (or interiority/exteriority, objectivity/subjectivity). I would not say this is a "middle path", but perhaps it is!
Overall, Goertz seems sympathetic to Muntzer and Luther is generally portrayed negatively (though not exclusively so). Goertz also does a really good job of making the book understandable to someone who really has no knowledge of the era. Names of areas and people are thrown up, but their importance are often explained. I would recommend at least a cursory knowledge of feudalism as certain feudalist terms are used but not explained (e.g. seignorial) but this is rather rare and does not make the text inscrutable.
This seems like a pretty niche subject, but I would recommend this book to anyone interested in pre-Marxist revolutionaries.