A really quick guide
The hard data
That can mean a serious bump of your revenues (depending on your business model).
In some games, we've also affected the growth rate, even doubling its viral growth (e.g. See the attached 'Mahjohng Style Hong Kong' actual downloads image).
How does it work?
With geo-leaderboards you can have 2,000 groups of 10 players competing to be the TOP 10 players in their city, instead to having just 100 competing at a worldwide TOP 100.
That's an increase of 200x times in reach. And the best thing is that you can have both systems.
They work with any sort of scoring system (cumulative, temporal, ascending, descending, etc) and the UI is completely customizable.