Back in 2007, I took "Introduction to Proofs," the class at my alma mater that was designed to transition you from the computational type of math with which most people are familiar to proofs-based theoretical math. It is kind of a grab-bag course at that school, dabbling in proposition logic, (abstract) algebra, analysis, discrete math, etc.
I hated my professor, Dr. J.
He would talk to the class in this sort of angry way all the time. He barely graded our homework. One time he gave us a quiz and then, when he returned it, and someone asked him how to do one of the problems, he couldn't do it on the board. One time he mistakenly declared that "Fish only bite when the moon is full" would translate to "The moon is full implies that the fish are biting" instead of the converse.
Ugh. I hated that guy. He was ugly too.
I did hear from someone else that their proofs class had covered way fewer chapters than ours. So we covered a lot of material, and at the end had to write a paper (ugh!). I wrote about taxicab geometry, which was very interesting for me when I did it. (The taxicab metric has appeared in so many courses since then it's not even funny - including this year in topology.)
Over the years, my hatred of Dr. J faded. I'd see him in the halls or elevator and he was always friendly to me. I guess he remembered me, probably as a good student. He was happy that I was going to grad school. For all I know he was a nice guy after all.
And in the meantime, time after time, these weird "obscure" (to me at that time) topics that we covered in that class came up. When I took my second non-Euclidean geometry course in undergrad, the abstract algebra we'd done in Proofs was really useful. Having covered (very well) the definitions of sets, relations, functions, etc., made them so much easier to understand in future classes. Cardinal numbers didn't show up again until my second semester of grad school, so it's great that I learned about them in there. The Peano axioms showed up in the first semester of grad school and nowhere inbetween.
Did I mention I really did not know the first thing about writing math (as opposed to logic) proofs when I started that class, but totally learned how?
Man, that class was awesome. It was easily one of the most useful classes I ever took. From an outcomes perspective, Dr. J was brilliant. I bet he was one of the best Proofs professors at my school. I bet he still is.
All of this is to say that my feelings about a professor can have very little relationship to how much I am learning or how useful the course is. This is not the only example of me hating someone and then thinking later that they were pretty great, or that I learned a ton. What it tells me is that I should put a lot less stock in my own feelings in situations like this.