You probably saw them a few times already: little fixed tabs positioned on the left corner, sometimes at the right bottom corner of the screen, that open a feedback form in a lightbox or pop-up when clicked. SEOMoz had one of these last time I checked, among several other sites.
What is your opinion on them, as visitors and site owners? Useful? Annoying? When would you add one of these to your site?
I like the tab. It isn't too in the way, and it looks very web 2.0.
I never click them and I don't like them. I would add one to a site if I get desperate.