The best way to get feedback on a product is to send a broadcast to the order list for the product and ask for it.  This only works when you also provide a way for them to respond.  Just asking for a reply is not going to get the results you're looking for.

You can use a survey service like www.SurveyMonkey.com.  They have both free and paid accounts.  You can get excellent results using the free service. 

You setup the survey to include the responses you want.  Be sure to ask pinpointed questions for them to answer - this will help you improve your product and also get ideas for additional products.  At the end, ask for a recommendation and a testimonial. 

This works for me every time!