I wrote this little Widget for one of my sites to display some of my favorite comments in the sidebar. I've release it here on WordPress.org because I didn't see any other plugins that do this simple task. If you have the Comment Karma on your favorite comments then just set the Karma value in the Widget settings to match otherwise this plugin just queries all the comments with the default Karma of 0.
I plan on adding a few more features when I have time but the idea here is to keep it simple: Display some rotating comments as Testimonials.
Let me know if there is something not working for you.