Professor Castella has a very heavy accent, so most of the time I have no idea what he is saying (but since the things he write on board make sense, it's fine). Most of the examples he use in class comes from the textbook, so if you don't understand just go read the book. His homework questions are a little bit more challenging than the textbook questions, but those questions really do increase your understanding of the concepts.
I had Brown for Math 31A and I got a B because Brown is very conceptual (his tests are very challenging for me, and they look nothing like the textbook questions), however professor Castella's tests are very fair, there are no surprises and he doesn't really try to trick you. I got A on both midterms (the class average for both is around 90).