I wouldn't call myself a leaderboard junkie, or even highly competitive when it comes to high scores, but I do enjoy seeing how I stack up compared to others in a game. And in those rare occasions where I'm actually pretty good at a game, it can be fun to put your best effort out there for the world to beat, and then trying to maintain a position at the top.
However, one major issue when it comes to leaderboards on Game Center is that they're extremely easy to hack. Why people cheat just to have some astronomical score put them at the top of a leaderboard is beyond me, but it's been a big problem for pretty much the entire existence of Game Center. It was a major problem on OpenFeint as well, but not nearly to the extent that it is on Apple's online gaming network. Because of this I pretty much relegate my leaderboard activity strictly to people on my friends list, and ignore the global leaderboards, which is a shame.
Today Apple has taken a step in addressing this problem by giving developers the ability to directly view and manage the top 100 Game Center leaderboards in their game. Previously if there was hacked scores or some other problem with a leaderboard, the developer would have to actually contact Apple and have them fix it. Now developers themselves can delete fake scores or block certain players where appropriate.
I'm sure this won't completely fix the problem of people messing with online leaderboards, but it should curb it significantly and hopefully now Game Center can be enjoyed the way it was intended.