I found his writing interesting and fun. I agree with his (not really his but it's a great summary) thesis that geography is a major driving factor in our history and that guns germs and steel enabled (not caused) European dominance.
There were a few times he appeared too committed to his argument or reductionist to fully acknowledge flaws or painted with too broad a brush and made no effort whatsoever to examine if there are any significant cultural contributions or the impact of wise or foolish great leaders. With his guns germs steel premise in mind, there Is no particularly good reason the middle east should not still lead/dominate the world like it did for the bulk of ancient history.
I found his declaration earlyish in the book (it's been a few years) that the native new Guinean(?) people he spent time with were actually on average more intelligent than typical Europeans to be extremely unlikely if not demonstrably false.
The "continent orientation" argument I found a bit limited also. While I concede that there is a larger temperate zone in asia, North America and Africa are truly enormous and have huge latitude stretches that would certainly allow domesticated crops to be transported over vast areas.
I also think he too quickly dismisses some north American and African megafauna. Yea zebra's are ****s, I'll give you that one. But bison are not at all a species that couldn't be ranched. It's not as if wild horses, oxen, or aurochs aren't a bit ornery and incredibly dangerous when provoked.