How effective, exactly? Offerpal says developers that use it saw three times as much offer-based advertising revenue. According to the white paper, most of that increased revenue comes from users that had never participated in an advertising offer before (they made up 93% of those who participate in the in-game promotions). The in-game offers are relatively sticky as well, with 10% of these new promotion users coming back to complete another promotion within a week.
Of course, OfferPal has a vested interest in putting a good face on in-game offers, since they provide those very offers to game developers as their core business. Still, we have no specific reason to doubt their reporting. If in-game offers are really this good at drawing more money out of social game players, you can expect to see them in a LOT more games relatively soon.