I've been using this plugin for almost 2 years, with just one repeated send problem, which was fixed the following day by an update.
I've set up the plugin to send an autoresponder upon subscription, and I send periodic newsletters to my subscribers.
The plugin is not perfect: the interface could definitely be more streamlined, and recently I started to miss the ability to put a captcha field (or a honeypot as someone suggested) in the subscription form. The lack of such feature is causing me the minor hassle of periodically checking subscribers and deleting fake ones.
But since the developer is working for free and keeping the plugin up to date, I have only praise for this plugin, and for its developer.
PS. Please note that my website is on a low-frills server, so if your installation is not working, it might be your provider's fault, or just incorrect configuration.