The ability to send out an email blast with custom content that is relevant for each recipient, not just one generic message like most other services do! I wish there was more of it available so we could use their service as much as possible without having too many users in our database at once or being limited by monthly limits when using some paid plans. It's very easy to set up campaigns quickly but also allows you to customize them easily if needed which makes things even better since no two customers are alike - they all have different needs/wants etc.. For us this has been great because now everyone can be reached individually instead of sending mass emails where people get annoyed due to lack of personalization (which happens often).