Prof Rubin is a very nice professor both as a lecturer and individual. The material is hard, and the lectures (which are slides based and supplemented with research papers) are very dense. However, the professor is always willing to give feedback and more insight on topics. He's a very responsive professor via emails and office hours as well.

The hardest part would be the increasing difficulties of the homework. The class' grade is based on homework assignments only (we had 6), and the first one or two are pretty easy and straightforward. However, they do get denser as the material gets rolling. Allocate your time carefully because the homework can be time consuming!