UChicago's rating system was quite useful, actually. The survey is given before the final, so exam difficulty doesn't matter. Professors who deliver insights you really learn from get the high ratings and those who don't (aka: they dwell in detail instead of creating understanding, can't communicate, read slides, make you fall asleep, or teach outdated material lose out). Over the years, professor ratings and course bidding points became a much better indicator of the quality of a course section than the description of a class.