Great idea for a plugin. I've been searching for something similar to Microsoft's KB articles that ask you if the article was Helpful or not.
I can't believe a plugin like that is not available for WordPress yet.
Yours is the closest I could find. However, the button alignment is off.
Also, I think it would be helpful for everyone if we could customize the color of the button and also change the size of the feedback options. I think if I dug around in your .CSS I would be able to find it and figure it out.