The ease at which you can customize your filters so that they pick up just what we want them too! It's not perfect as some things get left behind but it works well for us most times.
Sometimes when using certain filter options with multiple addresses I have had issues where my entire address base disappears overnight leaving me without any contacts/subscribers until refreshed again later. We are trying out many different methods of keeping our lists clean & useful while still being able to provide good service to potential leads who request information via emails sent through these services.