The current unofficial Constant Contact Add-On has bugs, and the developer does not seem to be actively concerned about keeping that plugin up to date. I was wondering why Rocket Genius does not simply create their own official add-on for this?
Currently the main beef with the unofficial plugin is that it will not allow for registration to more than one list. At all. Not only can you not sign up for more than one list at once, but if you sign up for any list using the plugin, it is the only list you end up on - it removes you from all other lists if you happened to be on one already.