I've installed Kampyle on my site (using a custom icon as their default choices are pretty bad - animated gif anyone?) and I've already had quite a few pieces of feedback. The great thing is that I've been able to fix a few bugs people have alerted me to and then have Kampyle email them back to let them know that the issue has been resolved.
Kampyle is pretty good as it is, but the thing that is putting Kampyle on the map is their recent integration with Google Analytics which means you can see all of your feedback stats within your Google Analytics account. The way they have done this is particularly clever - You need to download the Grease Monkey Firefox Extension and install the Kampyle Grease Monkey script, but then when you go to your Google Analytics account you'll see the Kampyle stats showing up in their own dashboard widgets. Pretty clever really. This was a major incentive for me as it means I can now see all of my stats in one place rather than having multiple accounts across the internet.
So, if you'd like to leave any feedback on the site just click on the "Leave Feedback" banner in the bottom right hand corner!