I haven't used Realm before, but after watching this talk I'm definitely going to be checking it out. It seems like could be a pretty solid replacement for Core Data. Fast, simple API, thread safe, works on both iOS and Android - a recipe for success.
It's funny - I signed up for Realm.io's mailing list a while back to get notified when they post a new iOS talk to their site (It's how I heard about the Andy Matuschak's "Refactor the Megacontroller" talk, for instance). But up until today, I didn't even realize that Realm's main gig was this local storage technology.