Mila Kunis and Justin Timberlake are a lot of fun together in "Friends with Benefits," with some real chemistry and a rapid-fire repartee that never gets old. It’s a romantic comedy that makes fun of romantic comedies, and the stars keep. Dylan (Justin Timberlake) is done with relationships. Jamie (Mila Kunis) decides to stop buying into the Hollywood clichés of true love. When the two become friends. Sure she can…