It is added to by a combination of people: project leads, team members and sometimes just a brain dump format. It pulls the items for the board from github issues and displays them in the waffle.io format.
So far, it has been really well received and people looking to contribute one-off type of stuff can look through it and pick something to complete and push to github.
We find that the greatest success in getting people to return and become active is to get them accomplishing something quickly. They come wanting to contribute and when it takes us a couple of Hack Sessions to get them working on something we inevitably lose them.
We haven't been using it long enough for it to get old. It is still a shiny new toy. I will say that it is awesome that you can submit an issue straight from the waffle.io dashboard. So for our less technical members that are testing apps and such, they can go straight there and put in the issues to be fixed. On a side note, I've been using it with clients for the past month and I love that aspect. No more emails with issues and no more explaining how to fill out a github issue.