One thing we’ve invested heavily in at MailChimp is our API. We try extremely hard to keep it updated as frequently as "MailChimp proper," and to keep it open and well-documented (mailchimp.com/api/rtfm). It’s one reason there are so many integrations and plugins out there for the chimp.
In the recent v4.3 upgrade, the API was upgraded to v1.2.2. We added full support for creating autoresponders and grabbing their stats via the API. Also added full support for the content importing options found in the app (import via URL, via ZIP, etc) and a slew of other stuff I’m not even going to pretend to understand (hey, APIs are for geeks only). More details, and a link to the changelog, can be found in our API Google Group.