The first half of the book fleshes out the historical collapses of several different cultures, from Easter Island to the Maya to Viking Greenland. I found most of this fascinating, and Diamond's contention that environmental causes played a role (often a dominant one) in all of these collapses is compelling.
The next section was perhaps even more interesting, as it described the civilizations that managed to survive, even when faced with the exact same problems as had destroyed the previous civilizations. Take Tikopia, for example. It's a six square mile dot out in the Pacific, but it has supported 1,000 people for thousands of years, longer than the Roman Empire lasted. One of the decisions arrived at by the ruling families in around 1600 AD was that the pigs on the island were disturbing agriculture and eating too much food. So all the pigs on the island were killed for the common good. The Tikopians gave up a valued luxury, possibly ensuring the survival of their civilization. There are other success stories too. Some led by the people, who see what's happening to their area - others led by 'kings' who can see the whole picture and make wise choices.
Finally, Diamond draws his attention to the modern world. This I found the least successful part of the book. It flops around hither and thither with occasional interesting sections (such as how some oil and mining companies are finding environmentalism profitable) but it doesn't have much focus.
One of the things that struck me when making my own comparisons to previous collapses and the world of today is that so many of the collapsed societies were physically isolated from other civilizations. The Easter Islanders eventually cut down all their trees, and thus there was no way to even build a boat, much less cross the 1,500 miles of open ocean to the next nearest speck of land. One may think that modern society is no longer so isolated, but face it: the Earth is a speck of land in the middle of void much greater than the Pacific Ocean.
Overall, I found Collapse inferior to Guns, Germs and Steel.
If you're an illiterate Angeleno, take note -- The Natural History Museum has put on a quick knock-off exhibit: Collapse?