When used, Bonds can be redeemed for 14 days of play time, eight Squeal of Fortune spins, or 160 RuneCoins. You can also redeem them for RuneFest tickets (although it takes more than one Bond for that). In a video from Jagex CEO Mark Gerhard, the genesis of Bonds is explained -- this is the latest step in the game's long-standing battle with gold farming accounts, offering players more control over membership costs while cutting out the need to pay money to third parties. So spend cash on improving your in-game fortune or spend your in-game fortune to save some cash, it's up to you.
[Source: Jagex press release]