One of the biggest features of MailChimp is "groups". Without groups you're required to create separate lists to segment customers which then have to be merged to remove duplicates. This can be very burdensome especially when you're dealing with a large amount of users. In our case we have 10k+ and each has a special customer type. If we send the wrong email to the wrong list or group than it can cause a rash of unsubscribes or complaints.
I guess what I'm getting at is, please please add group functionality to the Mailchimp plugin. I'm having to do really awful tasks and hacks to make it kinda work. It hasn't been updated since June and that was simply to update the API and remove bugs. This is the #1 reason we updated to developer and it's become this biggest headache. We would really appreciate a refresh.