On the Origin of Species is one of those books everyone's heard of but relatively few people have read. I think it's easy to forget how revolutionary Darwin's ideas were, because he presents them so logically and so thoroughly thought out, with examples derived from almost every major branch of science in Darwin's day, from embryology, from climatology, even geology. The idea of natural selection seems so obvious and so elegantly simple that it's hard to believe no-one thought of it before.
Of course, some of the science in the book is outdated - no mention of genetics, of course - so it's probably of little practical use to the modern-day science student. But it's still worth reading for its elegance and thoroughness in showing natural selection at work everywhere in the world.
Why, then, have I given it two stars? Well, it's a little, dare I say it, dull. Darwin uses so many examples to illustrate the same point - and he wishes there were space for more! - that the book is also fairly long. Happily, the last chapter, "Conclusion and Recapitulation", gives the argument of the book in potted form if you don't want to spend a month of your life on it.