Has anyone implemented an online feedback or usability form that you'd
consider successful? "Successful" as in, generated at least some minimally
useful responses while remaining unobtrusive to users?

I'm being asked about getting such a thing going on our library and digital
collections sites. But I'm hesitant on the value. All the examples of this
kind of thing that I've seen (e.g. various flavors of pop-up) or that have
been suggested seem annoying, or will be ignored, or will be annoying AND

Ideally I'd like to hear about:

1. Ways of gathering online feedback that have worked
2. Ways of gathering online feedback that have definitively NOT worked

Thanks for your thoughts!