We used it before switching over to Mandrill.
The main advantages for us is to free our own hardware of mail handling, to get better and more advanced statistics. (Mandrill) and to ensure our email is sent to our client and members inbox and not spam folder.
You reach a certain point that it is more cost efficient to outsource certain services, due to manpower and hardware costs, we reached that point when our outgoing emails passed 4000+ emails.