Hi,
I had one of these and I could completely feel it work. I mean, it helped me get the most complete crunches ever.
I had to sell it, though, because I got SO SO seasick on it. Oh, I was very nauseous, and couldn't use the equipment.
And, I never get seasick. .. it was just this thing.
Anyways, I was on a different board and asked if anyone else got seasick and they said no. So, if you're interested, get one. . I think they work very very well.