There's a decent amount of replayability to the game, though I know that ultimately completing the challenges isn't necessarily worth the effort. Though it's really interesting when you do. The contrast between playing challenges vs. regular missions is amazing just in terms of the players you're paired with. Challenge players have got their shit together, know exactly what to do, and won't at all frustrate you. I only accidentally did a set of them just now, but I got paired with two awesome people with a ton of costumes.
... I gotta admit, I'm hooked on wanting to unlock costumes now. Thank goodness I can't pay real money for them.