I think I've reached the point with weighted crunches where the cons of doing them with a 100lb dumbell on my chest for 12-15 reps exceeds their pros - it doesn't seem great for my lower back or chest, plus it's awkward trying to get into position.
I do back extensions for lower back, sidebends for obliques and weighted crunches were my main exercise for abs...now I need something else. Any recommendations? Hanging leg raises? Ab wheel? I've got gravity boots - inverted sit ups?