I would like to say that your service is unbeatable compared to your competitors.
...having found your service by chance, we've been completely happy and pretty surprised actually. You offer a lot of features, with a very reasonable & logical rate system. ...How much your service will help is possibly beyond your understanding, but I'm sure you've heard that from other people using your service.
I have been using Microsoft List Builder previously. You product is excellent. I will be cancelling my List Builder at the end of this month!