We had a very similar feature in our Magento Go platform, but since moving
to community we really missed this functionality.
The extension is very good value for money. It's simple to install, works well and has a great support. You'd get a new menu under customer profiles where you add or subtract balance, notify customer if needed, including a possibility to edit the email it sends out in "Transactional Email" part of magento. The money well spent...