The UI is lovely, the site is quick and lightweight and without a ton scripts running or connecting to other sites(which is a huge plus for privacy).
I love email notifications so I can get the rank information without needing to log in. The email tells you what the keyword rank currently is and what it previously was.
The chart is pretty and nice to look at, informative, and uses an intelligent legend which makes it great to decipher what line is actually where, things some other rank trackers are dearly missing. The fact that Yandex is included is a great addition.It also tracks Youtube.
You can view SEO stats, Social stats, combine Analytics to your ranking, and add notes to keep certain ideas|changes in mind and up to date. You can get Reports in .PNG|.PDF|.XLS, and White Label is available. It really looks great. If you click "Share" you get an HTML version of the report.