The ab-roller thing is very good for abs, but can be dangerous for the lower back if you're not very careful about form. It is also always beneficial to work any muscle group from different angles, so if oyu do continue with the ab-roller thing, don't neglect more traditional crunches and leg lifts on occasion.
# of reps is kind of a personal choice. I know people that do 3 sets of 30 and I know people that do 5 sets of 100... just make sure you push it until it burns, and even past the burn, if you can. That's the fastest way to tone.
__________________
Keep your body lean, your blood clean and your mind sharp. -Rollins
|