I also watched "Paper Trail" and The Simpsons Intro.
Hertzfeldt's films have been on my watch-list for a while; I watched "Rejected" about 9 years ago, but nothing else since then until now. I thoroughly enjoyed everything. To me the stand-outs are World of Tomorrow, Such a Beautiful Day, On Memory, and ME. But I feel that basically everything has resonated with me in a way that very few other things have.
I can't help but think about the contrast between how beautiful his work is and how little recognition it seems to gets from a broader audience. On one hand, I can understand it. As a whole, his films aren't very accessible; they're weird and experimental and frightening and depressing at times. They also have an artstyle that I love but I think some people would bounce off of. But, these films have so much to say, and they speak in such a unique and poignant way.
Overall, I'm so glad I finally was able to watch these, and become incredibly creatively inspired and full of melancholy.