Contrary to the other replies here, Calculus has indeed changed over time, or at least how it's taught. In modern calc texts, there is an emphasis on limits and formal proofs. In old calculus texts, in particular this book, there seems to be an emphasis on intuition over theory.

The question is a difficult one: Is a self-taught student putting themselves at a disadvantage by using an old text as their primary source of knowledge? It's a difficult question because it can only be answered by someone who's read both an old text and a new text, which seems rare. But here's hoping that the HN crowd features someone who's done just that.

For one thing, Newton barely makes use of calculus in the textbook and instead opts for geometric methods. And even with that, he is not rigorous at all because rigor hadn't been discovered yet. I wouldn't even put it on the same level as modern textbooks since so much had yet to be discovered about calculus.

I would not recommend the Principa to anyone except for historical interest. Everything in that book has been said more concisely by other people in the three hundred years since it was published.

I find Robinson's book pretty hard going, especially for a relatively introductory class, and would be very interested to hear whether or not this impression held for you.