For it being his first time teaching, Barr was decent. He's very enthusiastic, passionate, and invested in our learning which is nice having a professor that cares. Also, he provides a lot of resources for practicing the material and provides students with opportunities to get help in terms of office hours, campuswire, review sessions, and being open to questions during lectures. He's quite helpful when you ask him questions directly through audio and patiently and thoroughly explains the concept until you understand it to your satisfaction.
A lot of the negativity towards him comes from his insanely difficult midterms, which is fair as they are very difficult. However, his final, which was still challenging, was significantly easier than the midterms and has a much greater affect on your overall grade, giving you a chance to improve you grade significantly even if you did poorly on both midterms. Nevertheless, you do need to put in a lot of work for the class, as there is a lot of material to learn, understand, and master. It is quite a difficult course which can be a little disheartening at times but you definitely learn a LOT and in the end it works out better than expected.

The man is cool to talk to but thats about it. I found it difficult to understand the material with him as the professor. At the beginning of the quarter he explained, "How does one succeed in my class." Let me tell you, I did everything he asked for and still got a D on both midterms(individually at least , there is also a group portion that acts like a "curve"). I kid you not, I have no idea how I got an A- because i thought i was going to get a C+ or B- best case scenario. I think I must've done really well on the final(doing better on the final exempts one of your midterm grades). At the end of the day, this class is not worth the headache and stress. Find a different professor.

It's clear Barr is excited about teaching and his lectures are well put together, but the structure of his class is a little annoying and difficult at times. The worst aspect is that there are no answer keys to anything, so you need to make time for office hours if you want to check your work, and even the TAs don't often understand the problems he writes. The midterms were challenging and the wording of the questions made it even harder. The final was much more straightforward and there were a lot of opportunities for credit in other areas besides the tests (homework, discussion, etc).
Making time to go to office hours and completing his problem sets early so you can study from them makes this class doable.

Avoid at all costs. Makes 20B harder than it should be.