It is time again for student evaluations and the biannual ritual where students are given the authority to judge how well they were treated in their classes. While I agree that it is useful to have some indicators about the quality of teaching, I do not think students are the best people to ask about this.
As Walter Bossert argues, the facts that students are obviously no experts in the taught material, that they perform the evaluation anonymously without having to justify their marks and they have no guidelines on what the marks are worth makes this a highly dubious effort. Imagine if teachers were evaluating students this way!
From my own experience and from pouring through others' evaluations (a sad exercise), students reward those who make it easy on them. Just look how students discuss their teachers on Professor Performance or Rate My Professors. A committed teacher who wants her students to really learn and pushes them to work hard is doomed.
Then, how should teachers be evaluated? By their peers, and by students who have graduated. These are the people that can best evaluate how the teacher masters the material and how it has an impact.