I am going to agree with Benny!
Crunches to failure means you're going to start getting sloppy after around 25 good solid crunches. Furthermore, if you are holding a 20 or 25lb plate behind your head (I do this exercise) you are at best going to get 20 reps or so before your stomach feels like it's going to explode. The best results come from the best reps. As soon as your reps become less than good, you're probably pretty fatigued and starting to incorporate other muscles that have nothing to do with your abs.
One of the first things people start to do is with their hands behind their heads, they start pulling their head up. 1. Bad for your neck. 2. At what point do your abs and arms have any correlation in that position? NONE!
Under control reps with weight (resistance) is by far the best and fastest way to experience growth and cuts in your abs.