I never really did attend his lectures, but ended up with an A. He has a heavy Spanish accent. But he is unquestionably a great professor, and even better, one of the nicest professors you'll ever see. He is really friendly, is EXTREMELY prompt at responding to emails, and even nicer when you go to office hour. He's just too good a person. The tests were quite fair. Although, do not confuse them with easy. Basically if you do your work, you'll get what you deserve. Just to tell you how efficient he is at his work, he graded the entire 31B final in less than 8 hours, and even posted the final grades.
I am a HUGE fan of this guy, and he is my absolute favorite. A very pleasant person to talk to, a very nice and helpful human being, efficient at his job, and above all a great professor. Take him.
(Again, this class is not extremely easy, but at the same time, it is more than quite possible to get a great grade in his class)

I enrolled in Castella's class because my first pass was too late and at first I was skeptical of him because of his heavy accent. Gradually, I started liking him because I actually understand what he is teaching in class as opposed to the troubles I had during 31A.

His tests are fair. The test score will reflect the amount of effort you put into it. I scored well on both his midterms. Overall, I would recommend taking him

Really good professor. Knows exactly what to teach and how to teach it. Anyone talking about his accent is just being pretentious cause it isn't that bad. He's a very good professor to have for math.