We've rolled this into production to replace the system we had been using (we were sending via Google Apps FYI) on a site with 11,000 WP users.
We've had issues of being spam filtered in the past, which we'd like to manage using a tool, such as this, and also being able to track hard and soft bounces (and deal with user accounts accordingly!) is something we'd like Mandrill to do.
Issues at present:
1) I believe we're being spam filtered for some messages. In fact, some system messages (not all) are being spam filtered by my own admin email address, which uses Gmail (Goole Apps for business) - despite me repeatedly saying "allow messages like these" and etc.
2) Some messages from custom plugins we have built / developed aren't sending using Mandrill (and are therefore not sending at all). These mails use the native wp-mail function, so its not clear why it ain't working?
I'd be interested in helping you get this plugged in more powerfully. For example, automatically handling users who have hard bounces on signup programatically (cutting out spam accounts, for example) would be a great feature for us and save us culling dormant inactive accounts each month (which is what we do at present).