The ability to send out emails in bulk at little cost is great! I have so far used it only as an informational tool but would love if they had more features like sending surveys/polls etc via their service too - which could save you money otherwise spent through other tools when creating them yourself.
Sending multiple newsletters per day (or week) has been very helpful since my business deals mainly online now days. It saves me time by not having to set up each newsletter manually everytime i want one sent across.