TL; DR : Worth reading, if you can take it as speculative sci-fi rather than a history of things which haven't happened yet. Buy it if you're looking for something interesting and feel like stepping off your beaten literary path.
For some reason, it's become really popular to predict the coming fall of America. China is rising, we're falling, our age of dominance is over and we'll soon be playing but a bit part on the world stage. Depressing stuff. Which is why this book was, if not always completely believable, at least a refreshing read.
In this book, George Friedman first claims that he will draw a rough outline of how the next hundred years are going to play out, and then proceeds to draw a ridiculously detailed one. I won't pick apart all of his core assumptions here, because I would be in serious danger of breaking the strictly enforced 60,000 word limit. Suffice to say that, if you write a future history based on the assumptions that China and Russia will both fall apart, and America will continue to rise, you're building on shaky ground.
But let that go. Predicting the future is really a hopeless enterprise, and reading a book of predictions as a graven list of Things That Will Happen can only be annoying. That isn't to say, however, that The Next 100 Years isn't worth reading. Friedman starts with a valid (if not perfect) set of assumptions, and reasons forward along the most rational path - as far as any of history's twists can be called 'rational'. The result is a sprawling tangle of future wars and politicking that makes for a good story if you can get past the fact that you're supposed to take all of this seriously.
In the end, there's nothing more fun than reading about the US squaring off against Japan, and winning. Again.