Student Evaluations of College Professors

From NPR, about an experiment in Italy:

Here's what he found. The better the professors were, as measured by their students' grades in later classes, the lower their ratings from students.

"If you make your students do well in their academic career, you get worse evaluations from your students," Pellizzari said. Students, by and large, don't enjoy learning from a taskmaster, even if it does them some good.

There's an intriguing exception to the pattern: Classes full of highly skilled students do give highly skilled teachers high marks. Perhaps the smartest kids do see the benefit of being pushed.

The Air Force Academy found similar results a few years ago: 

In primary and secondary education, measures of teacher quality are often based on contemporaneous student performance on standardized achievement tests. In the postsecondary environment, scores on student evaluations of professors are typically used to measure teaching quality. We possess unique data that allow us to measure relative student performance in mandatory follow-on classes. We compare metrics that capture these three different notions of instructional quality and present evidence that professors who excel at promoting contemporaneous student achievement teach in ways that improve their student evaluations but harm the follow-on achievement of their students in more advanced classes.

Unless evaluations are overwhelmingly good or bad, I can't see how they're very useful in evaluating instructors--and even in those cases it appears that student evaluations could be of little value.