Storing achievement/leaderboard data in iOS

I'm starting work on a leaderboard and achievement system for my iOS game, and one thing all the documentation suggests is to save leaderboard/achievement data locally in case submitting it to the Game Center fails, and then the next time the game is able to connect it can send the leaderboard/achievement data at that point, so the user doesn't lose any data.

OK, but how do I store the data? Is NSUserDefaults good enough? Is there a better/more secure way to store the data?