Integration with MailChimp et al Is anyone aware of a solution for using Craft user data with something like MailChimp? This is a very simple use case of sending newsletters/announcements to registered Craft users. Would be even better if it could be by User Group, but I will settle for all users.

I understand that Sprout Email can connect to MailChimp mailing lists, but that does not address the issue of having to manually sync the Craft User data with a MailChimp list.

Thanks for any suggestions!
Most email service providers like MailChimp won't let you send to a list of addresses, even if they're user accounts on your site, without first adding them to a list inside the ESP's control panel.
If you're planning on sending out user emails through a tool like this, it's best to determine the sync strategy before you start signing up users.
That said you have a couple options for existing users:

Create a template to export your user information in CSV format, save it's output and import into MailChimp

Use the Campaign Plugin to send out messages