It provides the user with a bunch of interesting features, and it's quite easy to use.
Email scheduling works perfectly and it's accessible in a snap, while tracking is really a potentially powerful for firms who are concerned with the effectiveness of their messages for campaigning and selling (but it can be an addictive feature for the layman, too).
It supports the main email providers (i.e. Gmail, Outlook) and I can't complain with that, but of course users of other services might do (anyway, setting up the service with any generic provider is a quite accessible experience for the non tech savvy, too). Review collected by and hosted on G2.com.