Sorry, they don't. Please do not waste the money. Studies show that regular abdominal work on the floor or mat is more effective then any of the machines out there. A stability ball can be better, but only if it is used under the lower back, not the upper. Read my blog about stability balls to understand what size ball would be best for you.
Most people don't realize they are working their abdominal while doing other exercises, but they are. Correct usage of your abdominal is very important in every exercise that you do. Seek help if you are unsure.
-k
