I see a lot of websites with a fixed "Feedback" button stuck to the center right or some such place. My questions are:
a) Do people actually use this to provide feedback? Specifically, is there any data on usage of a button like this?
b) What kind of feedback, if any, should you expect?
Yes, people provide feedback in those. I have been responsible for public feedback on applications with millions of customers, so I've spent many hours reading such feedback.
Since the feedback is unfiltered and people can write whatever they would like, you should expect to get anything and everything. I've seen everything from personal threats to crash reports that were complete with logs and detailed discussions of what the user was doing before the crash. The vast majority of the feedback on these websites for the products that I have worked on has not been actionable. At best, I would take things like bug reports and ensure that they were in our tracking system, even if they were closed out as either duplicates or could-not-reproduce.
From a UX perspective, I haven't found that feedback to be necessarily useful. That said, whether it's worth it from a marketing perspective, or from allowing your users to feel like they're able to give feedback, might make it worthwhile. Even though I haven't found it useful, I still have always scrubbed it occasionally to ensure that I haven't missed anything. Very occasionally, I have written blog posts in response to feedback submitted through such a source.
