This book is possibly the most important book that has ever been published in the history of the world. (possibly as Maxwell's book is also in strong contention). It is difficult to sum the wide ranging implications that Newton had on the world, sure, everyone knows about gravity. But he did so much more - he proved Kepler's laws, showed how the binomial expansion worked, gave us the three fundamental laws of motion - which stood the test of time for centuries (until Einstein made a minor adjustment to them), he revolutionised optics and he invented the biggest powerhouse of the mathematical world: Calculus. (despite what the other reviewer says, Newton discovered it before Libnitz, although they discovered it indepentantly).
Obviously, not all of these things are in this book. In fact there is an entire separate volume dedicated to his work in optics. And yes, it is written in such a way to make it practically unreadable, so not one you can go to bed with.
However, purely on the earth moving implications of the work contained within these pages, it is my opinion that everyone should have a copy, if only to put on their shelf and be able to say: "I might not have read it, but it's the most important work that's ever been published. Kudos Newton"
Six stars if possible.