This class was the hardest CS class I've taken at UCLA, but this may differ substantially for others, because I chose to go solo, instead of with a partner, on the distributed systems group projects, which themselves were quite intense. The material is intensely rewarding: I went in knowing next to nothing, and left being able to pick apart production systems that are in use today, so I think the skills that this class teaches you are truly invaluable, and I would recommend this class to anyone that has an interest in it, but be prepared, because it's not going to be easy.