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!
First of all, it's a high quality, professional plugin, didn't have any bugs etc...I have used it on vBulletin and Xenforo boards. The newsletter never went to spam, I never got complaints about the newsletter. It's relatively easy to use and any time I had any issue Threadloom's support was very good and fast to respond. They have accurate analytics which you can confirm by comparing with google analytics. I have a board with 100k users and I was getting ~200 clicks from the newsletter everyday while only doing 1 newsletter monthly.