It is more mathematically rigorous with an emphasis on the theoretical fundamentals, especially compared to other online offerings that are more applied. It may not cover as many learning algorithms.
Here is a previous discussion https://news.ycombinator.com/item?id=4206445. And, a comment about the book https://news.ycombinator.com/item?id=6353736
And, here are some of the impressive comments about the professor:
> Yaser Abu-Mostafa was (by enormous margin) the most effective professor I had at Caltech. Despite being such an expert in the field, he understands clearly when a concept is particularly challenging--and what about it makes it so. [...]
>What Andy says is absolutely true. This machine learning class was easily the best class I took at Caltech. Prof. Abu-Mostafa got a standing ovation at the end of the course the term I took it. I wish I could have taken more of his classes. \ It was also fairly difficult -- the assignments were hard, but at every step, you could look at what you'd done and say "I know why I'm doing this, and I can see how this works." \ I remember at the end of the term he took several students' notes and made copies of them, so that he could compare the students' notes with what he was trying to convey, and could know if he wasn't teaching certain parts of the class well enough. [...]