Of all the performances, Urban's is the one I'm most curious to see.
I'm pretty certain Quinto will be great as Spock.
Pine doesn't have the option of just aping Shatner, so he'll have to create a somewhat new Kirk anyway. I'm optimistic that he'll pass muster, too.
But McCoy is so damn specific of a character, it's going to be a real challenge to re-create him.