This is more than a newsletter, this helps you find the best content on your forum and promote it! As a marketing tool for my forum, I will grade it as excellent. I've only been using it about a month and can see an improvement in daily number of visits and daily number of returning users.
As for usability, it has a lot of great features, and I hope they keep improving the interface and the documentation so that it can be even better. I would grade it as "Needs Improvement", keep up the good work!