First of all, I should say that I haven't read this book from cover to cover, but have dipped in and read it in several places. Nevertheless this has been enough both to appreciate the ambition of Watson's project and to see some of its failings. I am particularly interested in - and reasonably knowledgeable about - philosophy, and so started by reading Watson's accounts of various philosophers: Plato, Aristotle, Cicero, Descartes, Spinoza and so on. I was disappointed to discover that:
(i) Watson's knowledge of philosophy is patchy at best; for example saying, p.490 of my edition, that it was not clear why Descartes included appendices on meteors and dioptrics to his "Discourse on the Method". Well, I'm afraid it is clear to anyone who knows anything about Descartes: it was precisely as a demonstration of how his method could be applied, as his objective was to rebuild the sciences from scratch, rather than to construct a system of philosophy as Watson seems to believe. If you don't understand this you don't understand anything about Descartes' project. It seems to me here as elsewhere that Watson has leaned heavily on secondary literature rather than first-hand acquaintance with the sources which in this case are easy to read and would have enlightened him on this point.
(ii) what he does know is very much twisted to suit his objectives. For example, he gives a very biaised and rather dismissive account of Plato's thought, portraying it as "mysticism" when Plato was obviously more of a rationalist than a mystic. Plato is an immense thinker, and responsible for a huge step forward in rational thought - the development of the dialectical method and the concept, both of huge importance to subsequent thought yet barely if at all mentioned by Watson, who is however supposed to be writing a history of ideas.
These examples are from philosophy, something which I know something about, but this made me wonder if his coverage of other areas I am less familiar with can be trusted.
In another field, Watson affirms that "it is time to bury psychoanalysis as a dead idea, along with phlogiston, the elixirs of alchemy, purgatory and other failed notions that charlatans have found useful down the ages", (p.728 my edition) adding "It is now clear that psychoanalysis does not work as treatment". This is a highly subjective and controversial stance to say the least.
Watson clearly has an agenda, that of the no-nonsense British empiricist, prefering Aristotle to Plato, Locke to Descartes, which is fine, except that many people may be reading his work thinking that they are getting a reasonably unbiaised account of the history of thought, which I don't believe is the case. This is a pity as there is clearly a need for this kind of historical synthesis of ideas in today's confused world. So read this history, by all accounts: there is surely much of interest here, and perhaps Watson is stronger in areas other than philosophy, but do not believe that you are getting anything like an unbiaised account of the history of thought.