This is a fantastic tool that not only links Joomla registration to Mailchimp, but also makes writing newsletters super easy. Joomla artcles can be included in the newsletter, and you can see a report that shows statistics on the newsletter from the admin section of Joomla - never need to login to Mailchimp at all!
The bad news is that it will not work with Joomla 3 and development appears to be stone cold dead. I have delayed moving my site to Joomla 3 for over a year while hoping that someone would update this app. So far, no response - and I can't wait any longer.