First of all great plugin! It has made it stupid simple to integrate Mandrill with my sites.
However I have run into an issue, I am using the wpMandrill::mail function to send some emails because I need to pass along some metadata. I was adding my associative array but it was not showing in my activity. I checked the api logs and it was being sent, but it was being sent as "meta_data" instead of "metadata". I changed two instances of meta_data in the wpMandrill::mail function to "metadata" and now it is working.
Thought I should point it out, hopefully it will help others in the same situation.