I love how easy it is to use, you can create an email campaign in seconds without having any coding knowledge or experience! The best thing about mailbrainies though are their customer support team who helped me out when my account had been hacked (which was very rare).
Their interface could be better but that's just nitpicking - they have great content so there isn't much else to complain about here :) Definitely get them if you're looking into bulk sending emails through services such as Aweber/Mailchimp etc. They make creating campaigns super simple & help us send many different types of newsletters each week which helps our clients keep up-to date with what we do.