Easy to deploy, no configuration required; works well for our needs (sending out mass emails). The service has been reliable since we first deployed it about 4 years ago - which is not always true when you're trying something new! We use MAIB primarily as an SMTP relay that sends bulk messages from various systems through their own servers instead of using services like Sendgrid or Mandrill/MailChimp etc., so reliability was important but also cost factor because they charge per message sent rather than volume basis.