So I thought I should do my part as a Math major to tell y'all about Caputo, because I wasn't informed about him when I first came to UCLA.
I can tell that as a mathematician, he's very smart. He gives clear and good examples. His lectures are very thorough. The books he recommends are very advanced. He's made me more interested in Probability getting out of his class than when I first got in, and it's a shame that I didn't know him for 170A. (I had a horrible teacher for 170A and my knowledge was completely screwed up.) If you have him for Probability, I guarantee you'll have a more in depth understanding of the subject. Though I must warn you that he has a high expectation. His tests aren't so hard; they're all doable, I know that I screwed up in them because I'm not so smart when it comes to Probability. However, I still got an A, and it should show you how merciful he is.

Some people would feel a little uncomfortable around him, because frankly, he's easily irritated. Sometimes I also think that he's favoring some students more than the others. Just to warn some people who are super sensitive. But if you're like me, you'd only care about the material and how much you can get out of a professor, not his temper, you should definitely take him; he'll be around when you have questions.

Overall: if you want easy As ONLY, take Effros. If you want good grades (not easy As of course) and good knowledge, take Caputo. It's up to you.