Nicereply turns your email into a customer service feedback system
thenextweb.com
thenextweb.com
Having said that, I do love it when someone responds with a "Thank You". Or, in the case of one memorable client, poems praising how quickly the problem has been solved. <3 those.
For $20 a month you could sign up with an integrated feedback system which, if nicereply gains any traction, would soon integrate a similar feature anyway.