I am very impressed with the great attention to detail in this plugin. Everything is very well thought out, very thorough, and very solid.
My WordPress host is unique/odd in a number of ways, and I had concerns (among other things) about whether Wysija Newsletters would be able to use my host's special mail configuration without modification -- I needed to make a small modification the WordPress core to get WordPress itself (without the plugin) to send email, for example. But I had no problems with Wysija Newsletters at all -- the plugin anticipated everything, was very easy to configure, and worked the first time without modification.
It is a pleasure to use such a high-quality plugin. I plan on buying the premium version just before my site goes live.