In reply to SomeClimbingGuy:
Do you just want a strong core so you have a 6 pack or are you after a stronger core to get better at climbing? From my experience if it is the later i would sack off sit ups and other exercises not using the full chain of the core. I now do all my core training on a bar/fingerboard (leg raises, L hangs, front levers, wind screen wipers, jumping feet onto a chair placed way behind the fingerboard) or make sure that the exercises utilize the arms and shoulders i.e. dips with feet on a exercise ball, pressups (again can be done with feet on exercise ball to decrease stability). I find that these exercises transfer more directly to climbing where you are trying to transfer weight between your hands, arms, shoulders, core, legs, feet.