I have one and I love it... It completely works my abs more intensly. You are working as you raise and lower... I like the oblique work and the leg/glute work out as well.
I have only used it once at the gym so i obviouusly couldnt get results. But my cousin has 2 and her and her fiance do it together every day and they both swear by it.
Ooohh! Thanks for the posts. I just ordered one on monday. I did a search for the bender ball on the site and the results were not as positive. It just clicks for me. I think it will work. I can't wait to get it. I hate doing abs in the gym.