It depends where ur physique is at. If you are fairly lean(6-9%) and you can see your abs, these devices are a great way to supplement a workout.
I agree /w most of these guys here. You should correct the root of the problem, which is ur lower back. Focus rehabilating ur lower back through the use of a gymnastic ball or seek professional help.
Using the ball will offer u best ROM when done properly. Try doing weighted crunches and leg raises using one.