The MailChimp Integration Extension just received a major update. The entire extension was rebuilt from the ground up using the MailChimp 3.0 API. This makes things much more stable and useable, and will lead to a host of new features in the future. For now, most of the features stay essentially the same.
If you are upgrading the MailChimp extension from a previous version to version 2.0, here are some items to keep in mind.
The information here primarily applies to upgrading the extension. If you are doing a new install, you can just review the documentation here.