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.