Asking for honest, anonymous feedback can be scary. Really scary. But over the years, I've learned that the benefits of getting real feedback far outweigh the possibility that you'll get slammed. Feedback really does matter.
Last year was the first time I had ever been a contract instructor and the first time I taught my Introduction to Computers for Arts and Social Sciences course. I tried some new things that most other computer science profs at our school don't do. I needed to know whether these things worked. The teaching evaluations student have to fill in aren't terribly insightful so I created my own more detailed, course-specific survey instead.
I was pleasantly surprised with the responses. Lots of students took the time to fill in the survey and provided both positive feedback and constructive criticism. A couple of disgruntled students slammed me. That made me sad for a day but in the end I managed to find some useful information from their comments (especially the fact that they didn't realize what the course was really about!). The feedback I got was extremely useful in planning updates to the course this summer.
I just released a similar survey for this year's course. Within half an hour of posting it (and not even announcing it) I already had a few responses. These students are awesome. So far everything has been very positive, and has shown that I've found a really good set of topics for this course. I'm actually hoping to take the survey results to our school director and make the argument that the course description should be updated to reflect these more interesting topics and that the course actually covers these topics the rest of the year, not just in the summer. The consensus among the students is definitely that this should happen, and a surprising number are even saying they are more likely to pursue computer science or programming courses (at Carleton or in the community) as a result of taking my course.
Finally, a huge benefit to soliciting feedback is that I have some great quotes to use in my teaching portfolio. I've been compiling great tidbits from all my workshops and courses and posting them on my website (haven't posted this year's quite yet). If nothing else, I recommend everyone finds ways to get these kinds of quotes out of students at every opportunity! You can always keep the negative stuff from public's view, after all. ;)