The ability to add dates or create custom calendars for multiple users is very useful; this also allows us as marketing managers to be able to see what other marketers are doing in our area of responsibility at any given time.
There can sometimes seem like there's too much going on inside one screen - it would benefit from being broken up into more manageable screens/screens that focus only on certain functions (e.g., send mass communication). It has helped with planning out who we're working with within an agency / client team so that all work flows smoothly during project management meetings etc.;.