We've moved our transactional email away from Mandrill to Mailgun. We had continued using Mandrill after Mailchimp deprecated the service awhile back, because the amount of credits we were offered essentially made it free.
However, following a couple weeks of frequent downtime and poor service transparency from Mandrill, we decided it was time to make the switch. It appears they no longer had any engineers with the ability to identify the core problems.
Mailgun has been more reliable, yet not as reliable as we expected. We still see issues a few times per week with the API failing when we attempt to make a call. The Reporting UI is way better.