A neat plugin that offers many of the capabilities you'd find in paid-for solutions. If you don't need state-of-the-art capabilities, I'd definitely recommend trying this plugin.
I used the plugin for a week or so, then switched to the paid "PRO" version, which supports attachments in outgoing emails - an important feature for me.
Importation of my data from Aweber into the free version worked smoothly and switching to the PRO version was seamless.
It satisfies all my requirements for an autoresponder (which admittedly are humble) and enabled me to dump Aweber.
While even the PRO version doesn't offer all the sophistication of Aweber, I got got tired of paying Aweber every month, and am finding I can do pretty much all I want with this option.