I've had very hit or miss experiences with old (very old) books on math and physics. I'm dangerously close to graduating with an undergrad degree in math and physics, so I wasn't entirely unprepared to tackle such books (I hope).
On one hand, I picked up Newton's Principia, and frankly I found it incomprehensible. From what I understand, his mathematical notation is entirely different from what we use today, and a lot of his reasoning is hidden in impenetrable text or absurd geometric diagrams. If you wanted to learn classical mechanics, there are several more modern books that would serve you better.
On the other hand, I've read some things by Euler and a few 19th century mathematical papers, and I found them clear and readable. Euler apparently popularized a lot of mathematical notation, so I suspect works subsequent to him would be a lot easier for a modern reader to understand.