Crunches never worked for me, hundreds of reps and never felt it the next day. Just a few reps on the ab wheel and it feels like I worked abs the next day. Huge difference.
The only crunch I ever felt was using a low cable with a rope attachment, laying on the floor with a roller pad under my lower back and using a higher weight.
each to his own. I Train abs superset style. very Little rest between sets. I sometimes use a crunch machine in the gym. One of the other Trainers in the gym do them with an ez bar and uses a reebok step board for his feet...that i have never tried.