It's very easy to use, has all the features you need and it syncs with Google Calendar so that makes everything super-easy! I can't think of anything specific but there are some minor things here & there (a few bugs) which could be improved like maybe adding more formatting options or allowing text input in different ways than just keyboard shortcuts.
If your looking for something simple yet powerful then this would probably meet your needs best. The simplicity allows me to get my work done without having to spend time figuring out how to do stuff. This also means when i am on holiday etc., its easier as well because all my appointments will sync up from google calender too! Also helps keep track of what meetings we have coming up at our company.