I read a great blog post by Adan Covati called: Quick And Easy Surveys. Adam makes the point that you don't need to add a form to an email but you can use links that make it appear like you've added a form. Here is the image he used:
To take this further, why not add a simple link in the footer of your email (or at the bottom) that says: "Did you find this useful? yes/no". The idea here is that some people may be reading your emails but don't feel that the email is applicable to them or are not interested in this topic. You can analyze this data by cross referencing the people who selected "no" with their past email activity and contact data (title, demographics etc...). You can do the same with the people that selected "yes". This may be another way to analyze the success of your email campaigns.
Try it out and see if it works on a campaign and then you can decide if it's something that you want to implement across all campaigns. I would start off with your newsletters.