As other editors comment, one of the massive strengths of this excellent book is the way that Diamond presents his thesis and then, in each chapter, hammers home how the evidence that he presents us with supports that thesis. There is a tremendous unity of argument that supports his claims to a large extent. This coherence is defintely a strength of this book, even if some of the leaps of logic that he makes do stretch the truth somewhat.
This book was pretty monumental when it first came out. I think that it's hard for any book that has been so scrutinized to stand up to every criticism, but I do think the arguments in the book are mostly well supported even though they are not totally convincing.
I think the book is very coherent in terms of its theme. Diamond keeps hammering away at the same point over and over, right? It's all about geography, geography, geography. That's what the first part of the question means. As far as the second part goes, I would say yes. At least he has for me. But the problem is that you either believe him or you don't -- there's no way to prove for sure that culture or race didn't play into European dominance. So I think he makes a good case, but I'm already inclined to think that way anyway. If I believed that culture mattered, I'm not sure I'd be convinced by what he says because there's not really (and can't really be) any solid evidence that absolutely proves culture was irrelevant.