It is often better to get feedback while the user is using the product that is what we recommend at http://featuremonkey.com/

When you send an email and ask user to give feedback they don't really have an incentive to give feedback, maybe giving Amazon gift cards etc.. can increase the likely hood of reply.

The placement of give feedback link can be really important.

Maybe keep in near places where you are more likely to have issues or you want feedback from.