Ever had to deal with email deliverability issues? This alone is a very good reason to go with a hosted service. The self-hosted software looks good but I'd never touch that with a stick. There might be cases where regulation/privacy rules makes the self-hosted system useful/required but in all other cases, do yourself a favor and use a hosted service.
You can still use Mandrill or Amazon SES for bullet-proof email delivery. This gem just helps you to organize and schedule your mailings within your app.