It's getting to be that time of year again: course evaluations. (I have written on this topic before.) Course evaluations are very important to faculty, especially assistant professors and visiting and adjunct professors. For assistant professors, course evaluations are the major metric that goes into determining if a professor's teaching is adequate. For a visiting and adjunct professor, course evaluations are the major determinant of whether or not the faculty member has a chance at receiving another employment contract.
While the importance of course evaluations are of less formal importance to associate and full professors (who have tenure), they still tell the faculty how the semester went and provide guidance for ways to improve the course. This information is also used by the area chair to determine teaching schedules and by the Dean's office in doling out pay raises.
In any case students should take the responsibility of evaluating professors seriously. Make an effort to attend class on the day that evaluations are given out. This is as true for students who have gone to every class period as it is for students who stopped attending class a long time ago. If you have not been attending the class because you didn't value the way the professor spent the class time, then you should definitely go to class in order to fill out a course evaluation. If you don't go to class and fill out an evaluation, then you might as well be giving the professor your full endorsement.
Every year several courses get much higher evaluations than they should get because so few students attend class. I have seen professors get a 4.5 in a course --- a pretty good evaluation --- but only 15 of 50 students have filled out the evaluations. This isn't a reliable sample, but it's the only one we have to go on, so we have to use it. It would be interesting to know how those 35 students who didn't fill out the evaluations would have scored the professor, but students, faculty, and the administration will never know.
If you think the professor did a very good job promoting learning during the semester, then give him or her a 5. If the professor was average, a 4 is appropriate. (Remember, an evaluation of 4.2 or above is generally considered acceptable by the faculty when reviewing teaching performance.) If the professor should have been better (for example, if he or she frequently got in the way of the learning process and didn't seem to attempt to improve), then a 3 or lower is appropriate (and disappointing to the faculty member, and a strong signal to the School).
You should not evaluate professors punitively. You should evaluate them all on the same basis, and you should complete the process thoughtfully. The evaluation is important to the faculty member; it is important to the School; and it is important to future students.