Let me preface this post by saying I do work out a considerable amount and I'm always out there giving the new 'it' thing a-go especially when it comes to fitness. I bought the Bender Ball several months ago. For me at least, I didn't take a liking to it. It hurt my lower back everytime I used it I think it's more of a pain in the a$$ to do than a normal workout (average crunches). I did use it to exercise my lower abs (sandwich the ball in between my ankles and raise my legs up and down, while slighlty lifting my upper body from the floor).
I am more of a stability ball/ BOSU ball person- and both equipments are so much more versatile. Hope this helps.