I've finished the first three chapters, mainly dealing with Polynesia, and I have to say this book is even better than Guns, Germs and Steel thus far. That could be because the story of the Polynesians is so appealing to me: While medieval Europe festered in a dark age, the Polynesians were busy colonizing the south Pacific, using canoes that many people today (like Jared Diamond) would be afraid to put one foot in, much less engage in month-long voyages of discovery. While the Vatican was busy insisting that stars didn't exist, the Polynesians were using stellar navigation, all possibly without a written language.
By the peak of their civilization, the Polynesians had created a complex trading network scattered across an area at least as long (east-west) as Canada. Then, of course, came the collapse of major hubs in the trading network, which sparked a domino-like collapse of the civlization. It's this collapse, unsurprisingly, that Diamond deals with. In the case of Easter Island and Mangareva, the collapse came from overuse of the land, deforestation and soil erosion. An additional wrinkle in the case of Mangareva was the nearby islands of Pitcairn and Henderson, who were dependent on Mangareva for imports. When Mangareva collapsed, Pitcairn and Henderson both collapsed as well, though local conditions were unchanged.
Really fascinating stuff.