You have excellent answers concerning anonymity.  Regarding how seriously they are taken, it varies widely.  In the faculties I have been involved in (except, I think, Yale, but my involvement was minimal there), the evaluation process was an informal, department-internal matter, and so how seriously professors take their evaluations is a metter of (i) department politics, and how secure the professor feels in the department, and (ii) conmscientiousness. 

I have known a professor who simply said that he did not care what his students thought of his courses, he knew what he was going to teach, and he wasn't going to hold any hands.  His evaluations were awful, I think mostly because his course was much tougher than the other courses, and as one of the strongest research professors in his department, this did not cause him any trouble that really bothered him.  I should say, he has an excellent presentation style, very clear, very logical presentation, and I respected his stance.