Well, I can say this is the best Mailchimp extension for Joomla! I miss only one feature, and it's really important for me : multiple lists.
I have different newsletters for the different categories on my site. So when a user wants to subscribe, he can choose between different lists (or choose multiple lists at once).
Something like this Wordpressplugin would be very nice! http://wordpress.org/extend/plugins/multi-mailchimp-list-manager/screenshots/
Thanks in advance