This plugin works but it's not very pretty and the styling options are limited. I'm pretty au fait with HTML and basic php but you need a degree in science to decode the guts of this plugin if you want to make significant changes. I only managed it by using the Inspect Element tool on Chrome and then searching all 43 files of the plugin to find the relevant tags. And even that wasn't as straightforward as it sounds.
I was doubly frustrated with the widget that doesn't have ANY options at all, making it almost impossible to style into your sidebar.
Still, it works, and Mailchimp appears to stay on top of issues rained in the support forum.