Most of all their control panel is very easy to use, just add your domain, add some DNS records and you're ready to go. They offer 10000 free emails every month, that's enough for regular emails. The log file is also very helpful to see where are problems. The support is very helpful and fast even if you run your account with free credits.
It's possible to receive emails while using Mailgun too. They don't have a mailbox, but you can forward the mail to your own email address. The spamfilter they are using for this service is much to low and many spam messages are forwarded too.
If you need a great SMTP transactional service, go for Mailgun. The first years I never paid a $ and got some great service. Tip! Add you creditcard to get more free options.
I use Mailgun for every email which is send from a website form to the owners email box. This way I don't need a mail server on each web server.