I have integrated OpenFeint (OF) to one of our games, which uses Cocos2d & Chipmunk.
(Not sure this is true to others or not, in our case, less than 5% of players submit their scores to OF).
Personally, I think OF is great, not only for holding the scores but also introducing other games, help increase sale.
However, there are some issues left, a bit lag while OF's pop-up appearing, 'welcome back xxx' or 'your friend' is online.
OF is written in objective-C++, when integrated with Chipmunk, still leave a lot of warnings: deprecated conversion from string constant to 'char*'. Currently we just ignore these warnings, it relates to some features we do not call.
I'm thinking, should riq create an OF-like platform which also introduce only cocos2d games to players ?