I think this is an excellent way to acquire paying customers AND will give many potential users the option to see if they can build a decent subscribership without having to front the $250 or more that would be needed for ListBuilder.
Your interest in user feedback, as well as responsiveness, goes a long way in my opinion of the site.
I did my first mailer with you guys last week, and It was one of the most succesful ones we've had in a long while. As a result best weekend we've had all year! Thanks!