First, I will say that Diamond's research is impressive, his style quite readable, and his 'Further Readings' section commendable. Anyone interested in world building, whether for a role playing game, a fictional background, or just for the hell of it, will find this book well worth the $15 US, as will anyone who just happens to be interested in the evolution of human societies.
But (you knew there was going to be a 'but,' didn't you?), Diamond makes his agenda all too apparent in his writing. This is both good and bad. Good in that the agenda is obvious, and thus easily allowed for in examining his conclusions, but bad in that a historian shouldn't have an agenda. It seems painfully obvious that Diamond set out to prove that the differing rates of advancement of various human societies was do to environmental factors and not varying capabilities of the people themselves. I happen to agree, but I would have more confidence in his arguments if I thought that Diamond didn't have a goal in mind when he began gathering his data. And he never ceases to tell the reader that less technologically advanced peoples are not inherently, individually, inferior to people from more advanced societies, the direct implication being that he feels the reader needs to be told this. Even after the end of the book proper, in the endmatter, after 451 pages of setting out his conclusions, Diamond cannot resist one last dig; "Anyone inclined to believe that New World food production and societies were limited by the culture or psychology of Native Americans themselves, rather than by limitations of the wild species available to them for domestication, should consult...." One of the more amusing instances of this preaching comes in the chapter on the evolution of African societies, when he is discussing the language families of Africa. "Concealed at the top of Figure 19.2 is our first surprise, a big shock for Eurocentric believers in the superiority of so called Western civilization. We're taught that Western civilization originated in the Near East, was brought to brilliant heights in Europe by the Greeks and Romans, and produced three of the world's great religions; Christianity, Judaism, and Islam. Those religions arose among peoples speaking three closely related languages, termed Semitic languages: Aramaic (the language of Christ and the Apostles), Hebrew, and Arabic, respectively. We instinctively associate Semitic peoples with the Near East." He then gleefully reveals that the Semitic language family originated in north Africa. "Hence it may have been Africa that gave birth to the languages spoken by the authors of the Old and New Testaments and the Koran, the moral pillars of Western civilization."
At which I had to blink and ask myself, "Yeah? And so?"
This sort of thing makes Diamond's own Eurocentric bias all the more amusing when it creeps out, as it does in the final chapter, when he points out that if the plot to assassinate Hitler in 1944 had succeeded it would have ended WWII a year early. Uh oh, Jared, forget about those Japanese and Chinese did you?
The endmatter provides what I think is a big clue as to the motivation and direction of this book; I strongly suspect that Guns, Germs, and Steel is intended as a direct response to The Bell Curve. I haven't read the latter book, so I can't comment in detail, but that's the impression I get.
It should be pointed out that throughout the main text Diamond is talking about the advantages that geography and the local ecology gave to Eurasian societies compared to the rest of the world. He does not discuss the rise of Europe (as opposed to Eurasia as a whole) in the 16th century until the Epilogue, where he attempts to brush the whole inconvenient dichotomy under the rug with a few vague and contradictory arguments. For all intents and purposes, this book only discusses the evolution of human societies though about the middle ages. People interested in the latter period would to well to look elsewhere (I've found The Wealth and Poverty of Nations and Guns, Sails, and Empires to both be interesting and informative).