We've just started to use it for oxwall.org web site. Tried to synch all active users with mailchimp and got failed with it.
Also noticed, that it's impossible to assign automatically into mail list if a user has been joined to the specific role associated with the mail list.
For example:
There are several user roles on a site: Standard/Advanced
And there are different email campaigns created on Mailchimp for different user roles.
Every time the admin perform a mass mailing for different roles, he must execute synch operation in plugin settings. (Also, that process could cause memory leaks issue. Because as more profiles registered with the role, as more server's resources is required to perform a synchronization)
The better to associate any specific user roles with mail list only once. And then, as every new user is joined to the role, then automatically assign him with the associated mail list on mailchimp.