Things like this influence stakeholder decisions down the road. I've been evaluating migrating to Mandrill for some time to give our designer more control over transactional emails. Now, it's unlikely that I'll do so.
(Worth noting, that I'm a happy long-time Mailchimp customer)
We used Mandrill for some time, then moved to SendGrid, and just recently have moved again to Postmark because we were seeing delivery issues. FWIW, Postmark has been fantastic.
(Worth noting, that I'm a happy long-time Mailchimp customer)