I just received the class reviews for my Randomized Algorithms class. Historically, I have found my reviews are bimodal. Some students describe me as their favorite professor (and end up taking all my classes). Some students describe me in fairly unflattering terms. This year was pretty much the same.
The most signal from the noise was worth hearing, although pretty much stuff I already knew. First, students would like more of my time. I apologize for being busy, I suppose. Second, it would have much better to have had a TA. I agree (you think I enjoyed grading problem sets???). Sadly, my first choice TA was (rightly) busy working on his thesis, and there wasn't really another appropriate graduate (or undergraduate) student. But going without a TA is not something I'll try again.
A lot of the rest of the comments are lost in the jumble that comes from teaching to a class that serves a wide audience, not just theory folks. Some thought it was too fast, some too slow. Some liked that we pretty much followed my textbook, some thought I should let people read the textbook on their own and cover other stuff. Most found the problem sets a "challenge", which I think is just fine.
Class reviews never answer the question I really want answered. Five years out, was my class worthwhile? Has it had any impact on your research/job/worldview/life? Or even if you don't see an impact, do you remember it as a worthwhile learning experience? I don't care if you come out of the class thinking I'm some sort of sadistic homework machine (but really, I'm not...), if a few years down the road you find it was all worthwhile.
So ex-students -- not just MY ex-students, but all ex-students -- if you have a class you particularly liked several years back, a class that sticks in your mind or a class where the material has proven really useful to you later on, please send an e-mail to that professor and let them know. Anonymously is fine. That's the kind of feedback that means a lot to teachers. And you can even do it if you had a course you particularly disliked -- at least I wouldn't mind hearing from students who years down the line still think that I did a terrible job if they have constructive contributions to offer.