The plugin looks and behaves great and creates really nice newsletters. My only complaint (and why I did not give 5 starts) is because it's a real pain to extend.
I wish there were more hooks I could plug into to extend its functionality. I'm using time.ly's all-in-one event calendar on my site, and one of my requirements is to be able to insert these events into the newsletter using the WordPress Posts popup. The problem with the out-of-the-box behavior is twofold: first, it sorts by WP publish date, not event date. Second, the WP post title alone isn't often unique (it's often the same name, just the details that are changing). So the out of the box behavior for me when I use the WordPress Posts option to insert into my newsletter is a bunch of items with the same name, sorted in an insignificant manner.
I understand that, since these are custom post types and provided by a different extension, that wysija simply can't support this behavior out-of-the-box. However, it would be so much better if it could recognize that these types of things (sort order, title display within the popup) might want to be extended by people like me using WP hooks. Right now, I have to hack the middle of existing SQL statements, insert boolean variables before and after the core code, and comment it with sections that I changed, and then perform manual updates and reconcile the source code at every release. Not ideal, and easily preventable.